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PROFESSOR: This Wednesday
will be the first

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celebration of learning.

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Test 1 on Wednesday, October 7,
you will write during the

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normal class time.

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So you'll have 50 minutes.

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And we want to have a little
bit of comfort here, so you

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won't be sitting
cheek to jowl.

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So before long I'll have
the room assignment.

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So some of you will
write in here.

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We'll have fewer people than
seats so that there'll be

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vacancies next to each person.

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And then some will write in a
few of the other locations,

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probably 26-100 and the gym
above the Walker Memorial.

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And we'll get that out to you.

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And next week on the 6th we
will have no weekly quiz,

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because enough celebrating.

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No point in testing you on the
6th and then on the 7th.

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There will be, of course,
the mini-celebration

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tomorrow, quiz 3.

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And I'll be available for office
hours later today.

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Oh, and the coverage, just to
remove the mystery, will be

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right up to the 7th
of October.

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I've been doing this for over 30
years and I've learned that

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in order to inspire interest on
the part of the student it

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really pays to have the coverage
of the celebration

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extend up to the lecture
before the celebration.

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Now obviously I'm not going to
drill deep on something I

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taught you on October 5, but
it would be a good idea to

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stay awake during all of the
lectures between now and then.

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So last day we talked
about ionic bonding.

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And ionic bonding occurs with
electrostatic attraction

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between ions that have formed
through electron transfer.

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And we saw the energy of the
ion pair given by this

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formula, where we have
Coulomb's law

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with the Born exponent.

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And then this is plotted.

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This is E at r equals r0, and we
learned that there were two

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terms. The attractive term,
which is the Coulombic force

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here shown.

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And then there's a repulsive
term, which results from

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electron-electron interaction
when the two lines get very,

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very close together.

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And this is taken
from your text.

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And I think they did a very nice
job here of illustrating

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as you go to high values of r
they're depicting that you

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have the ions separated by
considerable distance.

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And there's a certain
amount of stored

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energy, but not a lot.

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And then if you go much, much
closer than the hard sphere

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sum of the ionic radii, I think
they're depicting here

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that there's some squashing
of the electron clouds.

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And you can see that the energy
has gone way, way up.

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So this is an unfavorable
situation, meaning that the

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energy here is greater than 0.

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And there's a sweet spot here
at 236 picometers, which

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represents the ideal location.

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And that is the sum of the
radius of the sodium ion and

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the radius of the
chloride ion.

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And so you can see how
energy tracks.

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And if you go far, far, far
away to the point where

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they're at infinite separation,

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there's no energy stored.

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So everything makes sense.

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And then we said, well what
happens if we keep packing

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these things?

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We rationalized that they would
continue to do so and

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ultimately form a 3-dimensional
crystal.

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And so you can see there's a
lot of similarity between

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what's above and what's
down here.

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This has been written
for a 1-1 system.

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In other words, a cation plus
1 and an anion minus 1.

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But it could be mediated
by the valences.

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And what we have here is
the structure factor.

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This Madelung constant tells us
how much we get decrease in

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the energy of the system
by going to a

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3-dimensional array.

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So depending on the atomic
arrangement we'll have a

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different value of Madelung
constant.

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We saw that for sodium chloride
the value is 1.7476.

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And different crystals have
different things.

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And what determines the
crystal structure?

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It's a combination of
the size of the two

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ions and their valence.

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So what we saw for sodium
chloride, this

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is a structure type.

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So obviously sodium chloride
is sodium chloride crystal

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structure, but there is an
entire suite of compounds that

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have radius ratios and charges
that end up with a sodium

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chloride type crystal
structure.

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And then towards the end we
started looking at the

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Born-Haber Cycle.

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And the purpose of the
Born-Haber Cycle was to give

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us a sense of scale of the
various constituents in the

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formation of a crystal.

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And what we noted, the takeaway
message from the

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Born-Haber Cycle, is that this
enthalpy of crystallization,

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which is basically this
term here, is huge.

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It's huge.

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It was the big component of the
energy required to form

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the crystal.

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It's large and it is negative.

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So what I want to do today
is start by talking about

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shortcomings of the business
of ionic bonding.

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See, how did we get
to ionic bonding?

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We started with this idea
of octet stability.

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Octet stability was
the driving idea

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behind all of this.

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Octet stability, and in the case
of ionic bonding this was

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via electron transfer.

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And so that got us a long way,
but it has its limitations.

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So let's put up some new data.

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So suppose I look at compounds
like H2, N2, O2.

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Do these things form
ionic bonds?

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How does octet stability
play here?

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And so let's start by
looking at hydrogen.

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So if we took hydrogen and
started with atomic hydrogen

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and added an electron to it,
then we would form an anion

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known as H minus.

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And H minus looks pretty good
because it's isoelectronic

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with helium.

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So maybe this isn't going
to be so bad a day.

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But if we're going to have
a bond then we need

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to form an H plus.

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So let's do that.

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So that would be, then, H goes
to H plus, plus an electron.

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And that's really nothing
more than a proton.

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So that doesn't look
too appealing.

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That's probably a high
energy state.

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And besides, in the same
location at the same time--

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in other words, same
temperature, same conditions--

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half of the hydrogens have to
acquire electrons and half of

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the hydrogens have to
lose electrons.

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And that's not going
to happen.

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They're either going to have a
propensity for electron gain

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or a propensity for
electron loss.

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So it looks like ionic bonding
is not going to help us

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explain the formation
of molecules such as

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H2, N2, and so on.

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So who came to the rescue
in this case to get

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us out of the conundrum?

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G.N.

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Lewis.

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G.N.

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Lewis was actually born in
Weymouth, Massachusetts and he

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finished his PhD at
Harvard in 1899.

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And then, like so many Americans
of the day, went off

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to Europe and he postdoc'd
in Europe for a while.

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And then he came back and
got a job at MIT.

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00:08:01,410 --> 00:08:05,280
And he taught at MIT
from 1905 to 1912.

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And then in 1912 he was lured
to the West Coast where they

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were starting to establish the
chemistry department, the

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00:08:12,190 --> 00:08:14,200
University of California at
Berkeley, and he went out to

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Berkeley and that's where he
spent the rest of his career.

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00:08:17,860 --> 00:08:20,850
And we can speculate
why he went.

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Maybe he was fed up with
the weather here.

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Actually, today is one of those
few days-- write it

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down, because one of the few
lovely days in Massachusetts.

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So G.N.

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Lewis, what did he say?

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He said, well I've
got an idea here.

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He said, what if hydrogen
achieved shell filling not by

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electron transfer but
by electron sharing.

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00:08:45,120 --> 00:08:51,560
So he posited the idea of shell

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filling by electron sharing.

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This is in contrast to
electron transfer.

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00:09:04,710 --> 00:09:07,430
So let's see.

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00:09:07,430 --> 00:09:07,600
Oh.

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00:09:07,600 --> 00:09:09,100
There's an image of G.N.

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00:09:09,100 --> 00:09:11,460
Lewis.

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00:09:11,460 --> 00:09:13,030
He died, actually, on the job.

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00:09:13,030 --> 00:09:14,640
He came back to his
lab one day after

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00:09:14,640 --> 00:09:17,050
lunch and hit the floor.

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00:09:17,050 --> 00:09:18,630
So he worked right
to the very end.

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Here's some data taken
from a lab

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notebook and memo, actually.

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00:09:26,950 --> 00:09:30,220
1902.

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00:09:30,220 --> 00:09:31,630
And what do you see here?

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Well, he developed a notation
for us, and we still use this

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00:09:36,280 --> 00:09:38,870
notation to this day:
Lewis notation.

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00:09:38,870 --> 00:09:41,160
So here's lithium and he's
got one electron.

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00:09:41,160 --> 00:09:43,810
But we know lithium has
three electrons but

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00:09:43,810 --> 00:09:46,540
only one valence electron.

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00:09:46,540 --> 00:09:48,600
And then there's beryllium
and magnesium--

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00:09:48,600 --> 00:09:49,470
two electrons.

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00:09:49,470 --> 00:09:51,360
Aluminum with three.

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00:09:51,360 --> 00:09:55,050
Here's fluorine chlorine, and
when they ionize he puts the

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00:09:55,050 --> 00:09:57,710
eighth electron right here.

195
00:09:57,710 --> 00:09:59,000
And look at this one
for silicon.

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00:09:59,000 --> 00:10:03,530
He's got probably some kernel
inside the atom, thus.

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00:10:03,530 --> 00:10:06,720
So he's even starting to think
about concentric shells.

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00:10:06,720 --> 00:10:07,890
This is 1902.

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00:10:07,890 --> 00:10:11,690
Remember the Bohr model
isn't until 1913.

200
00:10:11,690 --> 00:10:13,170
So you can see people
struggling.

201
00:10:13,170 --> 00:10:16,520
And notice that we have eight
electrons in a shell--

202
00:10:16,520 --> 00:10:18,500
that's where we're getting
the octet stability--

203
00:10:18,500 --> 00:10:19,740
and he's using cubes.

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00:10:19,740 --> 00:10:23,550
Now we know that the cube isn't
the shape of the shell,

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00:10:23,550 --> 00:10:26,160
but it's a pretty good device
to help you keep track of

206
00:10:26,160 --> 00:10:27,470
electron number--

207
00:10:27,470 --> 00:10:29,350
because there's eight
corners on a cube.

208
00:10:29,350 --> 00:10:33,180
So it's another example of how
that's not what it is but it's

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00:10:33,180 --> 00:10:36,220
a really good model and it keeps
you out of trouble and

210
00:10:36,220 --> 00:10:37,880
allows you to go forward.

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00:10:37,880 --> 00:10:41,990
So this is going back-- way,
way back-- for G.N.

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00:10:41,990 --> 00:10:42,560
Lewis.

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00:10:42,560 --> 00:10:47,530
Now let's use this idea
and account for the

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00:10:47,530 --> 00:10:49,930
formation of H2.

215
00:10:49,930 --> 00:10:53,160
So here's hydrogen, and using
the Lewis notation we'll put a

216
00:10:53,160 --> 00:10:55,870
dot here for its one electron.

217
00:10:55,870 --> 00:10:58,740
And we'll bring in a second
hydrogen and we'll use a

218
00:10:58,740 --> 00:11:02,800
cross, or an x, to indicate
the electron

219
00:11:02,800 --> 00:11:04,450
from the second hydrogen.

220
00:11:04,450 --> 00:11:06,465
And now we're going
to double count--

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00:11:09,550 --> 00:11:11,070
in other words, double
attribute.

222
00:11:11,070 --> 00:11:14,660
These are shared electrons so
they count for both atoms.

223
00:11:14,660 --> 00:11:19,210
Double count the shared
electrons.

224
00:11:19,210 --> 00:11:21,610
And when you do so what
do you come up with?

225
00:11:21,610 --> 00:11:28,010
Well, the element on the left
has two electrons and,

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00:11:28,010 --> 00:11:31,550
therefore, is isoelectronic
with helium.

227
00:11:31,550 --> 00:11:32,110
OK?

228
00:11:32,110 --> 00:11:33,410
Maybe it was a California
thing.

229
00:11:33,410 --> 00:11:34,650
They were sharing.

230
00:11:34,650 --> 00:11:36,350
And then there was
sort of another

231
00:11:36,350 --> 00:11:37,820
California concept, like.

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00:11:37,820 --> 00:11:39,100
So it was like helium.

233
00:11:39,100 --> 00:11:43,660
And then on this side this
is also sharing.

234
00:11:43,660 --> 00:11:46,950
And it's kind of like helium.

235
00:11:46,950 --> 00:11:51,810
So now we've achieved the
stability of the filled shell

236
00:11:51,810 --> 00:11:53,690
by sharing the electrons.

237
00:11:53,690 --> 00:11:54,060
OK.

238
00:11:54,060 --> 00:11:56,960
And I think I even have
another slide of how--

239
00:11:56,960 --> 00:11:59,590
this is the more modern
version of it.

240
00:11:59,590 --> 00:12:00,610
Electron dot.

241
00:12:00,610 --> 00:12:04,260
So the nucleus and the
inner-electrons are contained

242
00:12:04,260 --> 00:12:07,290
inside the chemical symbol.

243
00:12:07,290 --> 00:12:08,800
And, actually, this goes
all the way to

244
00:12:08,800 --> 00:12:09,970
modern quantum mechanics.

245
00:12:09,970 --> 00:12:13,050
Density functional theory does
the same thing: lumps all of

246
00:12:13,050 --> 00:12:17,920
the inner-shell electrons plus
the nucleus into one piece,

247
00:12:17,920 --> 00:12:20,780
and then the valence electrons
are outside.

248
00:12:20,780 --> 00:12:25,400
And so starting in 1902 with
some little dots and crosses

249
00:12:25,400 --> 00:12:28,050
we go all the way
to DFT today.

250
00:12:28,050 --> 00:12:29,560
All right.

251
00:12:29,560 --> 00:12:31,280
So let's do another one.

252
00:12:31,280 --> 00:12:32,770
How about nitrogen?

253
00:12:32,770 --> 00:12:33,800
Let's try nitrogen.

254
00:12:33,800 --> 00:12:37,080
So when we going to nitrogen
we know the valence

255
00:12:37,080 --> 00:12:39,460
electron's 2s2 2p3.

256
00:12:42,540 --> 00:12:44,070
So put nitrogen here.

257
00:12:44,070 --> 00:12:48,120
One, two, three, four, five.

258
00:12:48,120 --> 00:12:50,250
Now these three electrons
here are

259
00:12:50,250 --> 00:12:51,630
according to the Hund rule.

260
00:12:51,630 --> 00:12:56,005
So it's px, py, pz, and this is
the 2s2 sitting over here.

261
00:12:56,005 --> 00:13:00,710
And I'll bring in a second
nitrogen, and there's its 2s2.

262
00:13:00,710 --> 00:13:03,370
2px, 2py, 2pz.

263
00:13:03,370 --> 00:13:05,070
And now what do I have?

264
00:13:05,070 --> 00:13:06,850
Look at the nitrogen
on the left.

265
00:13:06,850 --> 00:13:08,500
Two, four, six, eight.

266
00:13:08,500 --> 00:13:12,360
So the nitrogen on the left
feels as though it has access

267
00:13:12,360 --> 00:13:12,990
to eight electrons.

268
00:13:12,990 --> 00:13:15,970
The nitrogen on the right--
two, four, six, eight-- it

269
00:13:15,970 --> 00:13:19,900
feels as though it has access
to eight electrons.

270
00:13:19,900 --> 00:13:25,390
So both nitrogens are
isoelectronic with neon if we

271
00:13:25,390 --> 00:13:28,010
push on this concept of
electron sharing.

272
00:13:28,010 --> 00:13:29,620
Now there's a second
thing I want to do.

273
00:13:29,620 --> 00:13:33,230
It's to draw attention to
two types of orbitals.

274
00:13:33,230 --> 00:13:37,320
So these three orbitals in the
center consist of electrons

275
00:13:37,320 --> 00:13:38,700
that are shared.

276
00:13:38,700 --> 00:13:43,140
So these are going to be called
bonding orbitals.

277
00:13:43,140 --> 00:13:46,125
And "bonding" and "blue" both
begin with a "b", so I'm going

278
00:13:46,125 --> 00:13:53,900
to denote the bonding orbitals,
or bonding domains,

279
00:13:53,900 --> 00:13:59,790
as distinct from the nonbonding
domains in red.

280
00:13:59,790 --> 00:14:01,545
Red are nonbonding domains.

281
00:14:04,270 --> 00:14:06,045
Always two electrons
per orbital.

282
00:14:06,045 --> 00:14:08,360
They like to live in pairs.

283
00:14:08,360 --> 00:14:09,740
That's the way it works.

284
00:14:09,740 --> 00:14:10,320
OK?

285
00:14:10,320 --> 00:14:13,280
And so each one of these
pairs is a bond.

286
00:14:13,280 --> 00:14:18,240
So I can then write nitrogen
with three lines through it

287
00:14:18,240 --> 00:14:20,220
indicating I have
a triple bond.

288
00:14:20,220 --> 00:14:24,710
Three pairs of electrons,
three bonding

289
00:14:24,710 --> 00:14:25,970
domains, triple bond.

290
00:14:25,970 --> 00:14:30,910
This is all in formation
according to the concept of

291
00:14:30,910 --> 00:14:32,510
electron sharing.

292
00:14:32,510 --> 00:14:36,210
And Lewis coined a name for
the type of bond that is

293
00:14:36,210 --> 00:14:37,720
formed in this way.

294
00:14:37,720 --> 00:14:50,720
He said we get bond formation
involves cooperative use--

295
00:14:50,720 --> 00:14:51,970
sharing, cooperative--

296
00:14:56,540 --> 00:14:58,210
of valence electrons.

297
00:15:02,710 --> 00:15:07,220
So now we can take the "co"
symbol here and the "valence"

298
00:15:07,220 --> 00:15:16,310
here and come up with the term
"covalent bond." Covalent

299
00:15:16,310 --> 00:15:18,700
bond, thanks to G.N.

300
00:15:18,700 --> 00:15:18,970
Lewis.

301
00:15:18,970 --> 00:15:22,970
So, again, to make sure we're
very clear, ionic bond results

302
00:15:22,970 --> 00:15:27,620
from electron transfer, covalent
bond results from

303
00:15:27,620 --> 00:15:30,200
electron sharing.

304
00:15:30,200 --> 00:15:33,950
Now we can do this-- so let's go
to heteronuclear molecules.

305
00:15:33,950 --> 00:15:34,980
These are homonuclear.

306
00:15:34,980 --> 00:15:37,230
So let's go to heteronuclear
molecules.

307
00:15:37,230 --> 00:15:39,690
And so let's see.

308
00:15:39,690 --> 00:15:42,710
I've got some rules
up here, I think.

309
00:15:42,710 --> 00:15:43,450
Yeah.

310
00:15:43,450 --> 00:15:44,470
Drawing Lewis structures.

311
00:15:44,470 --> 00:15:46,940
So let's go to a heteronuclear
molecule.

312
00:15:46,940 --> 00:15:50,385
And I'm going to choose as an
example sulfuryl chloride.

313
00:15:55,560 --> 00:15:57,100
And I don't expect you
to be able to name

314
00:15:57,100 --> 00:15:58,230
these things on site.

315
00:15:58,230 --> 00:15:59,620
I will always give
you the name.

316
00:15:59,620 --> 00:16:05,090
I'll say sulfuryl chloride,
parenthesis, SO2Cl2, blah,

317
00:16:05,090 --> 00:16:05,980
blah, blah.

318
00:16:05,980 --> 00:16:07,180
OK?

319
00:16:07,180 --> 00:16:09,690
So sulfuryl chloride.

320
00:16:09,690 --> 00:16:11,560
I want to put up the Lewis

321
00:16:11,560 --> 00:16:12,970
structure of sulfuryl chloride.

322
00:16:12,970 --> 00:16:15,910
So center the element with the
lowest average valence

323
00:16:15,910 --> 00:16:16,880
electron energy.

324
00:16:16,880 --> 00:16:20,790
So it turns out that the average
valence electron

325
00:16:20,790 --> 00:16:22,160
energy stack like this.

326
00:16:22,160 --> 00:16:28,160
Sulfur is the lowest, then
chlorine, and then oxygen.

327
00:16:28,160 --> 00:16:30,440
This is this ranking of average

328
00:16:30,440 --> 00:16:31,600
valence electron energies.

329
00:16:31,600 --> 00:16:33,900
And you'd be given those data.

330
00:16:33,900 --> 00:16:36,730
So it says put sulfur
in the center.

331
00:16:36,730 --> 00:16:38,640
So I'll put sulfur
in the center.

332
00:16:38,640 --> 00:16:40,610
And then what does it say?

333
00:16:40,610 --> 00:16:42,760
We're going to count all
the valence electrons.

334
00:16:42,760 --> 00:16:50,010
So sulfur over here
is 3s2 3p4.

335
00:16:52,780 --> 00:16:55,200
So that gives me six
valence electrons.

336
00:16:55,200 --> 00:16:56,930
And there's two oxygens.

337
00:16:56,930 --> 00:17:01,900
And oxygen lies above sulfur,
so that's 2s2 2p4.

338
00:17:01,900 --> 00:17:03,545
So that's 2 times 6.

339
00:17:06,620 --> 00:17:08,526
So that's 12.

340
00:17:08,526 --> 00:17:10,680
All right?

341
00:17:10,680 --> 00:17:12,350
Let's put the 6 over here.

342
00:17:12,350 --> 00:17:15,550
And then there's chlorines
in this compound,

343
00:17:15,550 --> 00:17:19,310
so that's 3s2 3p5.

344
00:17:19,310 --> 00:17:23,490
So that's 5 plus 2 is
7, 2 times 7 is 14.

345
00:17:23,490 --> 00:17:25,700
And we add this whole
thing up, we get

346
00:17:25,700 --> 00:17:28,910
there's 32 valence electrons.

347
00:17:28,910 --> 00:17:31,710
And draw a single bond from each
surrounding atom to the

348
00:17:31,710 --> 00:17:32,995
central atom.

349
00:17:32,995 --> 00:17:35,840
All right.

350
00:17:35,840 --> 00:17:36,950
Again, this is a model.

351
00:17:36,950 --> 00:17:39,270
I'm not saying that this is the
shape of the molecule, but

352
00:17:39,270 --> 00:17:40,010
it's a way to count.

353
00:17:40,010 --> 00:17:41,760
All I'm doing is trying
to keep track of

354
00:17:41,760 --> 00:17:43,640
bonds and paired electrons.

355
00:17:43,640 --> 00:17:47,900
So I can put chlorine
on either side.

356
00:17:47,900 --> 00:17:55,030
And I'll put an oxygen below
and an oxygen above.

357
00:17:55,030 --> 00:17:57,150
All right.

358
00:17:57,150 --> 00:17:59,770
So that's already two,
four, six, eight.

359
00:17:59,770 --> 00:18:03,350
So I'm losing eight.

360
00:18:03,350 --> 00:18:06,340
So 32 minus 8 is 24.

361
00:18:06,340 --> 00:18:10,990
And so with the 24 that means
I've got 12 pairs of

362
00:18:10,990 --> 00:18:14,560
electrons to place.

363
00:18:14,560 --> 00:18:16,410
So let's start putting the
Lewis structures up.

364
00:18:16,410 --> 00:18:20,780
So chlorine consists
of one, two, three,

365
00:18:20,780 --> 00:18:23,050
four, five, six, seven.

366
00:18:23,050 --> 00:18:24,780
And I'll do the same thing
on the other side.

367
00:18:24,780 --> 00:18:27,300
Two, four, six, seven.

368
00:18:27,300 --> 00:18:31,390
And oxygen has six, so that's
two, four, six.

369
00:18:31,390 --> 00:18:32,780
And then the lower
one, same thing.

370
00:18:32,780 --> 00:18:35,060
Two, four, six.

371
00:18:35,060 --> 00:18:36,340
And sulfur has six.

372
00:18:36,340 --> 00:18:37,810
I'm going to use
x's for sulfur.

373
00:18:37,810 --> 00:18:41,160
So I'll put one x with
the chlorine,

374
00:18:41,160 --> 00:18:43,280
another x with the chlorine.

375
00:18:43,280 --> 00:18:46,220
Two with the oxygen, two
with the oxygen.

376
00:18:46,220 --> 00:18:49,080
And so now we're in pretty
good shape, right?

377
00:18:49,080 --> 00:18:51,590
We can identify bonding and
nonbonding domains.

378
00:18:51,590 --> 00:18:53,920
Here's the bonding.

379
00:18:53,920 --> 00:18:58,160
One, two, three, four.

380
00:18:58,160 --> 00:19:00,440
And then the nonbonding.

381
00:19:00,440 --> 00:19:02,560
Looks like there's 12.

382
00:19:02,560 --> 00:19:10,435
And sure enough, 1, 2, 3, 4,
5, 6, 7, 8, 9, 10, 11, 12.

383
00:19:10,435 --> 00:19:12,070
The 12 nonbonding domains.

384
00:19:12,070 --> 00:19:13,130
4 bonds.

385
00:19:13,130 --> 00:19:14,760
And we have the Lewis
structure for

386
00:19:14,760 --> 00:19:16,490
this particular compound.

387
00:19:16,490 --> 00:19:19,660
And one last little piece
worth pointing out.

388
00:19:19,660 --> 00:19:23,450
Notice that in the bonds to
the chlorine you have two

389
00:19:23,450 --> 00:19:25,390
electrons, as you need.

390
00:19:25,390 --> 00:19:27,740
One electron comes from the
chlorine and one electron

391
00:19:27,740 --> 00:19:29,020
comes from sulfur.

392
00:19:29,020 --> 00:19:32,010
But in the bonds between the
sulfur and the oxygen the

393
00:19:32,010 --> 00:19:36,490
sulfur's so desperate to form
a bond that it actually

394
00:19:36,490 --> 00:19:39,720
donates both electrons
to the bond.

395
00:19:39,720 --> 00:19:43,180
And oxygen's happy because it's
isoelectronic with neon,

396
00:19:43,180 --> 00:19:45,830
and sulfur's happy because it's
going to be isoelectronic

397
00:19:45,830 --> 00:19:46,690
with argon.

398
00:19:46,690 --> 00:19:48,630
But, you know, it has to
go to some lengths.

399
00:19:48,630 --> 00:19:53,300
So this is called a dative bond,
when both electrons come

400
00:19:53,300 --> 00:19:56,510
from the one element.

401
00:19:56,510 --> 00:19:56,800
OK.

402
00:19:56,800 --> 00:19:58,100
Well, this is great.

403
00:19:58,100 --> 00:19:59,660
I'm going to do one more.

404
00:19:59,660 --> 00:20:03,550
How about methane?

405
00:20:03,550 --> 00:20:05,110
CH4.

406
00:20:05,110 --> 00:20:07,760
So I'm going to start
with carbon.

407
00:20:07,760 --> 00:20:09,330
Carbon's going to go
with this on there.

408
00:20:09,330 --> 00:20:14,220
And a carbon is 2s2 2p2.

409
00:20:17,600 --> 00:20:23,500
And so I'm going to use
the box notation now.

410
00:20:23,500 --> 00:20:28,590
See, this is the Lewis
structure, this is chemical

411
00:20:28,590 --> 00:20:31,450
equation, now we're going
to a box structure.

412
00:20:31,450 --> 00:20:34,040
We can move fluidly from
one model to another.

413
00:20:34,040 --> 00:20:35,670
We had cubes up there.

414
00:20:35,670 --> 00:20:36,620
It's all good.

415
00:20:36,620 --> 00:20:38,480
So this is 2s.

416
00:20:38,480 --> 00:20:44,130
And now this is 2px,
2py, and 2pz.

417
00:20:44,130 --> 00:20:47,420
Now according to this,
2s2, that gives

418
00:20:47,420 --> 00:20:49,450
me an electron pair.

419
00:20:49,450 --> 00:20:52,420
And now I've got 2p2, which
according to the Hund rule

420
00:20:52,420 --> 00:20:54,120
goes in like this.

421
00:20:54,120 --> 00:20:55,410
Well I've got a problem here.

422
00:20:55,410 --> 00:20:57,300
How many unpaired electrons?

423
00:20:57,300 --> 00:20:57,725
Two.

424
00:20:57,725 --> 00:21:01,635
Now what's my maximum number
of bonds I can form by

425
00:21:01,635 --> 00:21:02,886
electron sharing?

426
00:21:02,886 --> 00:21:05,170
It's two according to this.

427
00:21:05,170 --> 00:21:14,620
So the best I can do, best
possible here, is CH2.

428
00:21:14,620 --> 00:21:15,450
And that's no good.

429
00:21:15,450 --> 00:21:19,130
We know from mass measurements
it's CH4.

430
00:21:19,130 --> 00:21:21,230
The stoichiometry's CH4.

431
00:21:21,230 --> 00:21:24,530
And besides, what are these
orbitals going to look like?

432
00:21:24,530 --> 00:21:28,660
These are the p orbitals, so
they're dumbbell-shaped and

433
00:21:28,660 --> 00:21:29,850
they're orthogonal, right?

434
00:21:29,850 --> 00:21:33,200
They're 90 degrees, which means
if I formed this thing--

435
00:21:33,200 --> 00:21:34,650
which is called methylene--

436
00:21:34,650 --> 00:21:41,230
if I form methylene I'd end up
with CH2 looking like this,

437
00:21:41,230 --> 00:21:42,850
which has a dipole moment.

438
00:21:42,850 --> 00:21:46,070
And we know from spectral
measurements and electrical

439
00:21:46,070 --> 00:21:48,930
properties measurements that
this thing is symmetric.

440
00:21:48,930 --> 00:21:51,630
So this thing--

441
00:21:51,630 --> 00:21:53,430
electron sharing
isn't working.

442
00:21:53,430 --> 00:21:55,060
It's not working.

443
00:21:55,060 --> 00:21:59,790
So we need another patch here,
and that patch comes from none

444
00:21:59,790 --> 00:22:02,630
other than Linus Pauling.

445
00:22:02,630 --> 00:22:03,400
Another American.

446
00:22:03,400 --> 00:22:06,360
You see, it's American science
today and it's in the 20s.

447
00:22:06,360 --> 00:22:10,230
That's why we have Gershwin
playing at the beginning.

448
00:22:10,230 --> 00:22:11,780
Celebration of American
science.

449
00:22:11,780 --> 00:22:14,380
So Pauling was born in
Portland, Oregon.

450
00:22:14,380 --> 00:22:16,130
He was the son of a pharmacist.
And he went to

451
00:22:16,130 --> 00:22:18,990
Caltech, got his PhD in 1925.

452
00:22:18,990 --> 00:22:22,470
So the Rhapsody in Blue came out
in 1924 when he was just

453
00:22:22,470 --> 00:22:23,880
hunkering down to his thesis.

454
00:22:23,880 --> 00:22:26,390
Probably listened to it, got
some pleasure out of it, as

455
00:22:26,390 --> 00:22:28,110
most people did.

456
00:22:28,110 --> 00:22:32,970
So then he finishes, 1925, at
Caltech in Pasadena and he

457
00:22:32,970 --> 00:22:34,230
goes to Europe.

458
00:22:34,230 --> 00:22:35,440
Now he chose wisely.

459
00:22:35,440 --> 00:22:38,830
He chose four postdoctoral
positions.

460
00:22:38,830 --> 00:22:40,310
These are people he
postdoc'd with.

461
00:22:40,310 --> 00:22:42,990
First with Sommerfeld, then
with Bohr, then with

462
00:22:42,990 --> 00:22:45,220
Schrodinger, and then
finally with Bragg.

463
00:22:45,220 --> 00:22:46,100
You'll learn about Bragg.

464
00:22:46,100 --> 00:22:48,695
Bragg got the Nobel Prize
for X-ray diffraction.

465
00:22:48,695 --> 00:22:54,000
So that's not a bad
preparatory start.

466
00:22:54,000 --> 00:22:57,310
So he comes back and
teaches at Caltech.

467
00:22:57,310 --> 00:23:00,450
In fact, I have a picture
of Linus Pauling.

468
00:23:00,450 --> 00:23:01,070
There he is.

469
00:23:01,070 --> 00:23:05,520
That's a middle-aged Linus
Pauling, probably around the

470
00:23:05,520 --> 00:23:09,720
time he got the first
of two Nobel Prizes.

471
00:23:09,720 --> 00:23:10,910
So what did Pauling do?

472
00:23:10,910 --> 00:23:14,850
Pauling said, why don't
we mix the orbitals?

473
00:23:14,850 --> 00:23:17,510
They're all in the shell n
equals 2, and what we're

474
00:23:17,510 --> 00:23:21,050
trying to do is to fill
the n equals 2 shell.

475
00:23:21,050 --> 00:23:23,180
So how about mix?

476
00:23:23,180 --> 00:23:33,550
Let's mix 2s and 2p states
in order to maximize

477
00:23:33,550 --> 00:23:34,500
the number of bonds.

478
00:23:34,500 --> 00:23:36,740
Remember, when you form a
bond you decrease the

479
00:23:36,740 --> 00:23:37,880
energy of the system.

480
00:23:37,880 --> 00:23:40,090
Four bonds is a greater
decrease in

481
00:23:40,090 --> 00:23:41,260
energy than two bonds.

482
00:23:41,260 --> 00:23:43,500
So the system, if it could--

483
00:23:43,500 --> 00:23:45,355
and they're all within
the same shell.

484
00:23:45,355 --> 00:23:49,500
You notice he didn't say, gee,
if mixing 2s with 2p is good

485
00:23:49,500 --> 00:23:50,910
let's go get some 1s.

486
00:23:50,910 --> 00:23:53,450
Well 1s is down in n equals
1, and there's

487
00:23:53,450 --> 00:23:55,190
no way you can mix.

488
00:23:55,190 --> 00:23:57,460
They had to be in
the same shell.

489
00:23:57,460 --> 00:24:07,550
Mix states in order to
maximize number of

490
00:24:07,550 --> 00:24:10,235
bonds that can form.

491
00:24:12,800 --> 00:24:14,480
It's all about maximize
the number of

492
00:24:14,480 --> 00:24:17,325
bonds that can be formed.

493
00:24:20,630 --> 00:24:23,740
And this, of course, is
by electron sharing.

494
00:24:23,740 --> 00:24:26,350
We're talking about covalent
bonds here.

495
00:24:26,350 --> 00:24:26,920
OK?

496
00:24:26,920 --> 00:24:35,170
And he termed these mixed
orbitals "hybrids." Termed the

497
00:24:35,170 --> 00:24:37,050
mixed orbitals as "hybrid
orbitals." They're

498
00:24:37,050 --> 00:24:39,120
cross-breed--

499
00:24:39,120 --> 00:24:41,720
part s, part p.

500
00:24:41,720 --> 00:24:45,490
So now let's look at the
energy level diagram--

501
00:24:45,490 --> 00:24:46,590
or the box notation.

502
00:24:46,590 --> 00:24:47,330
Forgive me.

503
00:24:47,330 --> 00:24:50,160
So I'm going to mix s and p.

504
00:24:50,160 --> 00:24:53,720
So I've got a single s and I've
got three p's, so this is

505
00:24:53,720 --> 00:24:55,260
called sp3.

506
00:24:55,260 --> 00:24:59,695
Each one of these is a mixed
sp3 hybrid orbital.

507
00:24:59,695 --> 00:25:01,370
And I've got four of them.

508
00:25:01,370 --> 00:25:03,610
And how many electrons
do I have?

509
00:25:03,610 --> 00:25:04,410
Four.

510
00:25:04,410 --> 00:25:07,270
So now I use the Hund rule
and in go the electrons.

511
00:25:07,270 --> 00:25:10,090
One, two, three, four.

512
00:25:10,090 --> 00:25:15,260
And now I have the ability
to form four bonds.

513
00:25:15,260 --> 00:25:16,460
But it gets better.

514
00:25:16,460 --> 00:25:17,480
Here's the next thing.

515
00:25:17,480 --> 00:25:18,870
These are degenerate.

516
00:25:18,870 --> 00:25:19,900
They're all in the same state.

517
00:25:19,900 --> 00:25:21,770
That's why we're using
the Hund rule.

518
00:25:21,770 --> 00:25:26,390
And so degeneracy in energy
implies degeneracy in spatial

519
00:25:26,390 --> 00:25:28,050
orientation.

520
00:25:28,050 --> 00:25:28,920
So what does that mean?

521
00:25:28,920 --> 00:25:32,500
It means that if these are four
bonds equivalent, then

522
00:25:32,500 --> 00:25:35,560
the way those bonds will arrange
themselves in space is

523
00:25:35,560 --> 00:25:36,740
to be equivalent.

524
00:25:36,740 --> 00:25:39,970
So if I've got a central carbon
here and I'm going to

525
00:25:39,970 --> 00:25:43,480
put four sticks from the central
carbon so as to make

526
00:25:43,480 --> 00:25:47,200
the four sticks symmetrically
disposed in space, that

527
00:25:47,200 --> 00:25:50,110
dictates the architecture
of the molecule.

528
00:25:50,110 --> 00:25:53,130
And how do I put four sticks
off of a central point

529
00:25:53,130 --> 00:25:54,960
symmetrically disposed
in space?

530
00:25:54,960 --> 00:25:57,720
One, two, three, and four.

531
00:25:57,720 --> 00:26:00,640
This is meant to be the corners
of a tetrahedron.

532
00:26:00,640 --> 00:26:03,520
Each one of these is
109 degrees apart.

533
00:26:03,520 --> 00:26:07,060
And this describes
a tetrahedron.

534
00:26:07,060 --> 00:26:10,670
So now I've got carbon in the
center, and now I've got the

535
00:26:10,670 --> 00:26:14,900
hydrogens at the four corners
of a tetrahedron.

536
00:26:14,900 --> 00:26:17,810
There is the structure
of methane.

537
00:26:17,810 --> 00:26:21,080
And each of the hydrogens has
a shared electron with the

538
00:26:21,080 --> 00:26:24,040
carbon, making it isoelectronic
with helium.

539
00:26:24,040 --> 00:26:28,220
And the carbon has four of its
own electrons, four shared

540
00:26:28,220 --> 00:26:30,080
with the four hydrogens
to make it

541
00:26:30,080 --> 00:26:31,360
isoelectronic with neon.

542
00:26:31,360 --> 00:26:32,700
So everybody's happy.

543
00:26:32,700 --> 00:26:35,920
Shell filling, and
it's all good.

544
00:26:35,920 --> 00:26:39,800
So now it's symmetric and it
has no net dipole moment.

545
00:26:39,800 --> 00:26:43,180
Everything squares
with the data.

546
00:26:43,180 --> 00:26:45,330
Well, good for Pauling.

547
00:26:45,330 --> 00:26:46,000
But he went further.

548
00:26:46,000 --> 00:26:47,550
He went much further.

549
00:26:47,550 --> 00:26:50,890
What Pauling wanted to do was
to make it quantitative.

550
00:26:50,890 --> 00:26:54,020
And so he wanted to have
something analogous in

551
00:26:54,020 --> 00:26:58,100
covalent bonding to what we
have in ionic bonding.

552
00:26:58,100 --> 00:27:01,740
So what Leslie is now rubbing
off the board there is--

553
00:27:01,740 --> 00:27:02,390
no, keep going.

554
00:27:02,390 --> 00:27:02,670
It's good.

555
00:27:02,670 --> 00:27:03,490
It's OK.

556
00:27:03,490 --> 00:27:06,810
This is a rule of academics:
You always erase that which

557
00:27:06,810 --> 00:27:08,160
you will refer back to.

558
00:27:08,160 --> 00:27:10,330
We need more boards in here.

559
00:27:10,330 --> 00:27:12,430
How many boards do I
fill in a period?

560
00:27:12,430 --> 00:27:14,120
9, 18?

561
00:27:14,120 --> 00:27:14,580
Maybe what?

562
00:27:14,580 --> 00:27:16,580
We need about 24 boards.

563
00:27:16,580 --> 00:27:18,000
That's a good lecture.

564
00:27:18,000 --> 00:27:18,420
All right.

565
00:27:18,420 --> 00:27:21,760
So here's what Pauling
was thinking about.

566
00:27:21,760 --> 00:27:25,170
He was thinking about the
analogy, for example, if I

567
00:27:25,170 --> 00:27:32,240
want to get the energy of
magnesium oxide I can use the

568
00:27:32,240 --> 00:27:36,060
formula that Leslie has
just erased, and

569
00:27:36,060 --> 00:27:37,060
it looks like this.

570
00:27:37,060 --> 00:27:41,320
So if all I need to know is the
radius of the anion, the

571
00:27:41,320 --> 00:27:45,910
radius of the cation, it's
charge, and the Madelung

572
00:27:45,910 --> 00:27:47,870
constant and then I just
plug in, I get the

573
00:27:47,870 --> 00:27:50,030
crystallization energy.

574
00:27:50,030 --> 00:27:53,640
But then suppose instead of
magnesium oxide I want to go

575
00:27:53,640 --> 00:27:56,690
to magnesium chloride.

576
00:27:56,690 --> 00:27:58,910
I can use the same formula
only I need

577
00:27:58,910 --> 00:28:00,150
the Madelung constant.

578
00:28:00,150 --> 00:28:02,940
This is the Madelung constant
for magnesium oxide.

579
00:28:02,940 --> 00:28:04,560
If I have the Madelung
constant--

580
00:28:04,560 --> 00:28:07,150
forgive me, script M--

581
00:28:07,150 --> 00:28:12,790
for magnesium chloride, and
I know the ionic radius of

582
00:28:12,790 --> 00:28:17,540
magnesium cation and chloride
anion, away I go again.

583
00:28:17,540 --> 00:28:18,270
I need this.

584
00:28:18,270 --> 00:28:19,760
I need, of course, the
Born exponent.

585
00:28:19,760 --> 00:28:22,440
This Born exponent
and away we go.

586
00:28:22,440 --> 00:28:24,260
The same formula applies.

587
00:28:24,260 --> 00:28:28,100
So I can build with a library
of basic physical data.

588
00:28:28,100 --> 00:28:29,580
So what did Pauling do?

589
00:28:29,580 --> 00:28:32,470
Pauling said, what if we
can do the same thing

590
00:28:32,470 --> 00:28:34,340
for covalent bonds?

591
00:28:34,340 --> 00:28:37,250
Is there some kind
of an analogy?

592
00:28:37,250 --> 00:28:42,100
So he said, let's take a
look at an arbitrary

593
00:28:42,100 --> 00:28:43,310
heteronuclear compound.

594
00:28:43,310 --> 00:28:47,790
So I'm going to do this with
HF, hydrogen fluoride.

595
00:28:47,790 --> 00:28:50,680
So, first of all, let's build a
hydrogen fluoride molecule.

596
00:28:50,680 --> 00:28:54,090
H with its one electron, and
fluorine with its seven.

597
00:28:57,510 --> 00:29:01,900
So now hydrogen sharing an
electron with fluorine is

598
00:29:01,900 --> 00:29:03,790
isoelectronic with helium.

599
00:29:03,790 --> 00:29:04,470
It's happy.

600
00:29:04,470 --> 00:29:07,220
And fluorine sharing the
electron hydrogen is

601
00:29:07,220 --> 00:29:09,060
isoelectronic with neon.

602
00:29:09,060 --> 00:29:10,010
It's happy.

603
00:29:10,010 --> 00:29:17,840
So again we see shell filling
by electron sharing.

604
00:29:17,840 --> 00:29:24,020
So what Pauling wanted to ask
is, can I get a measure of the

605
00:29:24,020 --> 00:29:30,890
HF bond energy knowing
only the bond

606
00:29:30,890 --> 00:29:35,560
energies of H-H and F-F?

607
00:29:35,560 --> 00:29:38,240
So then if I knew all the
homonuclear bond energies and

608
00:29:38,240 --> 00:29:41,580
then I mixed these to make
heteronuclear bonds, is there

609
00:29:41,580 --> 00:29:44,050
a path from homonuclear
bond energy to

610
00:29:44,050 --> 00:29:46,560
heteronuclear bond energy?

611
00:29:46,560 --> 00:29:51,230
So let's look and see what
the numbers are.

612
00:29:51,230 --> 00:29:52,710
So hydrogen.

613
00:29:52,710 --> 00:29:54,370
The hydrogen bond's
fairly strong.

614
00:29:54,370 --> 00:29:58,200
It's 435 kilojoules per mole.

615
00:29:58,200 --> 00:29:59,780
That's mole of bonds.

616
00:29:59,780 --> 00:30:00,150
435.

617
00:30:00,150 --> 00:30:03,680
Fluorine-flourine is 160.

618
00:30:03,680 --> 00:30:07,040
And so what do you think
the value of the

619
00:30:07,040 --> 00:30:08,900
H-F bond should be?

620
00:30:08,900 --> 00:30:12,260
Well when I first look at this
I say, well it's part H and

621
00:30:12,260 --> 00:30:15,970
it's part F, so it's somewhere
between 435 and 160.

622
00:30:15,970 --> 00:30:18,140
I don't know if it's the
arithmetic mean--

623
00:30:18,140 --> 00:30:20,470
you know, add these two and
divide by two-- or maybe it's

624
00:30:20,470 --> 00:30:21,680
the geometric mean--

625
00:30:21,680 --> 00:30:23,710
multiply them together and
take the square root--

626
00:30:23,710 --> 00:30:25,630
but it's got to be somewhere
in between.

627
00:30:25,630 --> 00:30:27,040
What do the data show?

628
00:30:27,040 --> 00:30:33,460
The number's 569, which
is greater than 435.

629
00:30:33,460 --> 00:30:37,090
So I take a bond of 435 and
a bond of 160, I put them

630
00:30:37,090 --> 00:30:41,620
together I get 569.

631
00:30:41,620 --> 00:30:44,410
That's very, very strange.

632
00:30:44,410 --> 00:30:45,650
But Pauling was smart.

633
00:30:45,650 --> 00:30:48,250
Pauling said, I have
an explanation.

634
00:30:48,250 --> 00:30:55,200
He says, suppose when these
electrons are shared in

635
00:30:55,200 --> 00:31:02,260
between the two atoms, suppose
they're not shared equally.

636
00:31:02,260 --> 00:31:06,400
Suppose there is a displacement
of the electrons.

637
00:31:06,400 --> 00:31:08,620
So instead of putting them
dead center, as I've been

638
00:31:08,620 --> 00:31:12,400
doing up until now, suppose
the electrons are actually

639
00:31:12,400 --> 00:31:16,680
drawn closer to the fluorine.

640
00:31:16,680 --> 00:31:20,000
So we still have octet
stability, or in this case

641
00:31:20,000 --> 00:31:22,880
duet stability, but
the sharing of the

642
00:31:22,880 --> 00:31:25,440
electrons is not equal.

643
00:31:25,440 --> 00:31:27,425
So this is charge
displacement.

644
00:31:31,160 --> 00:31:33,795
And what does charge
displacement constitute?

645
00:31:33,795 --> 00:31:36,605
Well, charge displacement
means stored energy.

646
00:31:41,180 --> 00:31:45,930
And Pauling quantified
that stored energy.

647
00:31:45,930 --> 00:31:49,820
And so what he did is he said
that you increase the bond

648
00:31:49,820 --> 00:31:54,250
strength by thinking of it
as a two-step reaction.

649
00:31:54,250 --> 00:31:57,730
So in the heteronuclear bond
that is a bond between two

650
00:31:57,730 --> 00:32:07,240
different atoms. So in a
heteronuclear bond we form by

651
00:32:07,240 --> 00:32:09,970
what-- and this is my coinage,
you don't see this

652
00:32:09,970 --> 00:32:12,310
anywhere in the book--

653
00:32:12,310 --> 00:32:16,930
two-step, share and then pull.

654
00:32:16,930 --> 00:32:20,160
So share is, as the name
implies, we share electrons to

655
00:32:20,160 --> 00:32:21,670
achieve octet stability.

656
00:32:21,670 --> 00:32:26,550
But then because we have unequal
atoms we pull towards

657
00:32:26,550 --> 00:32:31,680
one of the atoms. And which
one do we pull towards?

658
00:32:31,680 --> 00:32:34,150
Well, we pull towards the one
that's got a greater appetite

659
00:32:34,150 --> 00:32:35,370
for electrons.

660
00:32:35,370 --> 00:32:37,720
And we've already gone
through this concept.

661
00:32:37,720 --> 00:32:40,720
Which atoms on the periodic
table have the highest

662
00:32:40,720 --> 00:32:42,770
appetite for electrons?

663
00:32:42,770 --> 00:32:44,110
The nonmetals.

664
00:32:44,110 --> 00:32:45,730
The weakest appetite
is the metals.

665
00:32:45,730 --> 00:32:47,430
The metals are good donors, the

666
00:32:47,430 --> 00:32:49,200
nonmetals are good acceptors.

667
00:32:49,200 --> 00:32:52,320
And fluorine's up in the top
right corner, so fluorine has

668
00:32:52,320 --> 00:32:54,830
a very, very high appetite
for electrons.

669
00:32:54,830 --> 00:32:57,680
And, indeed, in this bond
the electrons are

670
00:32:57,680 --> 00:32:59,660
pulled to the right.

671
00:32:59,660 --> 00:33:04,240
And why Pauling got the Nobel
Prize and Lewis didn't-- it's

672
00:33:04,240 --> 00:33:07,170
my theory-- is that Pauling
was quantitative.

673
00:33:07,170 --> 00:33:10,020
So he came up with a
quantitative measure.

674
00:33:10,020 --> 00:33:22,830
He devised a quantitative
measure for the degree of

675
00:33:22,830 --> 00:33:35,000
unequal sharing, thereby
allowing us to make these

676
00:33:35,000 --> 00:33:42,090
calculations with
some accuracy.

677
00:33:42,090 --> 00:33:50,620
And he called that quantity
electronegativity, and it's

678
00:33:50,620 --> 00:33:52,850
denoted by the Greek
symbol chi.

679
00:33:56,180 --> 00:33:57,000
OK?

680
00:33:57,000 --> 00:33:58,740
And he devised a whole scale.

681
00:33:58,740 --> 00:34:00,050
How did he get the scale?

682
00:34:00,050 --> 00:34:06,700
He looked at bond energies for
all sorts of pairs of elements

683
00:34:06,700 --> 00:34:10,110
across the periodic table and
went through an exercise with

684
00:34:10,110 --> 00:34:14,910
pencil and paper that today we
would call multivariable

685
00:34:14,910 --> 00:34:16,540
regression analysis.

686
00:34:16,540 --> 00:34:19,075
And came with a set of--

687
00:34:19,075 --> 00:34:19,420
[SLIDE APPEARING]

688
00:34:19,420 --> 00:34:21,150
PROFESSOR: Oh, I'll
come back to this.

689
00:34:21,150 --> 00:34:22,520
This is the structure
of methane.

690
00:34:22,520 --> 00:34:23,870
This is the s and p.

691
00:34:23,870 --> 00:34:24,810
Oh, let's take a break.

692
00:34:24,810 --> 00:34:25,950
You can stack.

693
00:34:25,950 --> 00:34:26,370
All right.

694
00:34:26,370 --> 00:34:27,300
So this is what methane
looks like.

695
00:34:27,300 --> 00:34:28,960
There's the s, there's the p.

696
00:34:28,960 --> 00:34:31,170
And the sp hybrid
looks like this.

697
00:34:31,170 --> 00:34:33,260
It's sort of an asymmetric
dumbbell.

698
00:34:33,260 --> 00:34:34,590
And these four things
stick out.

699
00:34:34,590 --> 00:34:35,684
And then you bond the hydrogens

700
00:34:35,684 --> 00:34:37,320
and there's the methane.

701
00:34:37,320 --> 00:34:37,960
OK.

702
00:34:37,960 --> 00:34:38,950
So here's what the

703
00:34:38,950 --> 00:34:41,100
electronegativity scale looks like.

704
00:34:41,100 --> 00:34:43,220
It looks a lot like the
scale for average

705
00:34:43,220 --> 00:34:44,550
valence electron energies.

706
00:34:44,550 --> 00:34:48,490
The nonmetals have the highest
appetite for electrons period,

707
00:34:48,490 --> 00:34:51,460
which means in a bond they're
going to hog the electrons.

708
00:34:51,460 --> 00:34:54,200
And the nonmetals have the
weakest appetite, and so

709
00:34:54,200 --> 00:34:58,040
they're going to end up having
the electrons in a covalent

710
00:34:58,040 --> 00:35:00,480
bond pulled away from them.

711
00:35:00,480 --> 00:35:04,750
So nonmetals have high
electronegativity, metals have

712
00:35:04,750 --> 00:35:05,605
low electronegativity.

713
00:35:05,605 --> 00:35:08,230
And now here's taken
from the text.

714
00:35:08,230 --> 00:35:12,600
And you see that the
electronegativity is periodic.

715
00:35:12,600 --> 00:35:16,090
If you go across a period the
metal has the lowest value and

716
00:35:16,090 --> 00:35:18,610
the nonmetal has the highest.
And there's fluorine, number

717
00:35:18,610 --> 00:35:21,220
nine, at a value of about 4.

718
00:35:21,220 --> 00:35:25,690
It's got the most intense
appetite for electrons.

719
00:35:25,690 --> 00:35:29,010
And then you jump down here to
sodium, et cetera, et cetera.

720
00:35:29,010 --> 00:35:32,260
Here we are going across the
lanthanides and whatnot.

721
00:35:32,260 --> 00:35:33,590
And this is taken
from your text.

722
00:35:33,590 --> 00:35:36,150
There's fluorine, 3.984.

723
00:35:36,150 --> 00:35:37,440
That's the thing.

724
00:35:37,440 --> 00:35:40,990
And down here we have very low
values of electronegativity.

725
00:35:40,990 --> 00:35:46,650
So with electronegativity
we are now able to make

726
00:35:46,650 --> 00:35:47,900
calculations.

727
00:35:50,140 --> 00:35:54,370
And this is the Pauling formula
for calculating the

728
00:35:54,370 --> 00:35:58,580
bond energy in a heteronuclear
bond starting from homonuclear

729
00:35:58,580 --> 00:35:59,280
bond energy.

730
00:35:59,280 --> 00:36:01,900
So let's continue with the HF.

731
00:36:01,900 --> 00:36:05,280
So if I want to get the
bond energy of HF

732
00:36:05,280 --> 00:36:06,430
I'm going to take--

733
00:36:06,430 --> 00:36:07,770
and this is the Pauling
formula--

734
00:36:07,770 --> 00:36:08,940
the geometric mean.

735
00:36:08,940 --> 00:36:12,940
So I take the bond energy of
hydrogen-hydrogen times the

736
00:36:12,940 --> 00:36:13,960
bond energy of

737
00:36:13,960 --> 00:36:17,700
fluorine-fluorine, and square root.

738
00:36:17,700 --> 00:36:20,290
So that's the geometric
mean of the two.

739
00:36:20,290 --> 00:36:24,360
And then comes the Pauling
piece that gets

740
00:36:24,360 --> 00:36:26,670
him the Nobel Prize.

741
00:36:26,670 --> 00:36:29,330
You take the difference in the
electronegativity between the

742
00:36:29,330 --> 00:36:37,220
two elements squared, and then
the factor 96.3 gives us the

743
00:36:37,220 --> 00:36:42,140
unit consistency with
kilojoules per mole.

744
00:36:42,140 --> 00:36:45,960
So the greater the difference
in electronegativity the

745
00:36:45,960 --> 00:36:51,110
greater the contribution here in
terms of the deviation from

746
00:36:51,110 --> 00:36:54,750
just the geometric mean of the
two homonuclear bond energies.

747
00:36:54,750 --> 00:36:57,730
Or put another way, if you have
a homonuclear atom such

748
00:36:57,730 --> 00:37:02,280
as H2, if it's chi H minus chi
H is 0, so this second

749
00:37:02,280 --> 00:37:03,620
term goes to 0.

750
00:37:03,620 --> 00:37:06,090
And obviously when fluorine is
one of the members you're

751
00:37:06,090 --> 00:37:07,540
going to get a very, very
high number, because

752
00:37:07,540 --> 00:37:08,730
this has the most--

753
00:37:08,730 --> 00:37:10,640
and it doesn't matter which
order you put them in because

754
00:37:10,640 --> 00:37:12,220
you're taking the square,
so it's always

755
00:37:12,220 --> 00:37:13,850
going to come out positive.

756
00:37:13,850 --> 00:37:17,220
And I want you to appreciate the
sense of scale here, so if

757
00:37:17,220 --> 00:37:18,750
we go in here we'll multiply.

758
00:37:18,750 --> 00:37:23,130
This is going to be
435 times 160.

759
00:37:23,130 --> 00:37:25,590
And I'm going to take the
square root of this.

760
00:37:25,590 --> 00:37:29,270
And then this is 96.3.

761
00:37:29,270 --> 00:37:30,830
And you look on your
periodic table

762
00:37:30,830 --> 00:37:33,040
this is 2.2 for hydrogen.

763
00:37:33,040 --> 00:37:35,250
Fluorine is 3.98.

764
00:37:35,250 --> 00:37:38,880
And I know there are different
tables of electronegativity.

765
00:37:38,880 --> 00:37:39,420
I don't care.

766
00:37:39,420 --> 00:37:41,750
Just whatever you've got
on your periodic table.

767
00:37:41,750 --> 00:37:44,580
The one in the book is a little
bit different but it

768
00:37:44,580 --> 00:37:47,060
all comes out in the wash.

769
00:37:47,060 --> 00:37:49,770
So you multiply all this out,
and we find that the first

770
00:37:49,770 --> 00:37:54,140
term is 264 kilojoules
per mole and the

771
00:37:54,140 --> 00:37:56,720
second term is 344.

772
00:37:56,720 --> 00:38:00,080
So this second term is even
greater than the first term.

773
00:38:00,080 --> 00:38:03,020
So the amount of energy in that
electron displacement is

774
00:38:03,020 --> 00:38:04,290
substantial.

775
00:38:04,290 --> 00:38:08,772
And if you sum the two
of these you get 608.

776
00:38:08,772 --> 00:38:10,220
Now you might say, well
wait a minute.

777
00:38:10,220 --> 00:38:15,030
The real number is 569.

778
00:38:15,030 --> 00:38:20,700
But 608 takes you in the right
direction and accounts for the

779
00:38:20,700 --> 00:38:22,610
contribution of electron
displacement.

780
00:38:22,610 --> 00:38:25,080
264 is just plain wrong.

781
00:38:25,080 --> 00:38:31,310
So this was an important
start for Pauling.

782
00:38:31,310 --> 00:38:35,050
And he has labels on these
two contributions.

783
00:38:35,050 --> 00:38:37,470
This first term, which is just
the combination of the

784
00:38:37,470 --> 00:38:44,150
homonuclear bond energies, is
called purely covalent.

785
00:38:44,150 --> 00:38:46,033
It's the purely covalent
contribution.

786
00:38:49,770 --> 00:38:53,430
And it's what I've been
referring to as the sharing.

787
00:38:53,430 --> 00:38:57,640
This is what you get
from sharing.

788
00:38:57,640 --> 00:38:58,080
OK?

789
00:38:58,080 --> 00:39:00,650
And then this second term here
with the difference in

790
00:39:00,650 --> 00:39:04,000
electronegativity is what you
get from what I've been

791
00:39:04,000 --> 00:39:08,050
calling the pull on
the electron pair.

792
00:39:08,050 --> 00:39:12,580
And Pauling called this the
partial ionic character.

793
00:39:16,392 --> 00:39:19,390
He's not saying that there's
electron transfer, but it's a

794
00:39:19,390 --> 00:39:21,320
move in that direction.

795
00:39:21,320 --> 00:39:23,430
Partial electronic character.

796
00:39:23,430 --> 00:39:27,460
So what I've done here is I've
decided I'll make a sort of a

797
00:39:27,460 --> 00:39:30,130
panorama of what we've
seen up until now.

798
00:39:30,130 --> 00:39:32,510
And so I'm going to make
something called the electron

799
00:39:32,510 --> 00:39:36,380
sharing meter.

800
00:39:36,380 --> 00:39:37,090
All right.

801
00:39:37,090 --> 00:39:41,530
So if I look at a homonuclear
system like hydrogen.

802
00:39:41,530 --> 00:39:45,240
So my meter reads neutral.

803
00:39:45,240 --> 00:39:47,220
So the arrow's at 12 o'clock.

804
00:39:47,220 --> 00:39:49,920
The electrons are
shared equally.

805
00:39:49,920 --> 00:39:55,070
And then if I go to
HF what do I have?

806
00:39:55,070 --> 00:39:58,850
Well, I know that the fluorine
is pulling the electrons.

807
00:39:58,850 --> 00:40:05,050
And so we can designate that by
writing delta minus, delta

808
00:40:05,050 --> 00:40:07,900
plus. delta the physicists
use.

809
00:40:07,900 --> 00:40:11,300
The lowercase Greek delta
means little bit of.

810
00:40:11,300 --> 00:40:11,970
All right?

811
00:40:11,970 --> 00:40:14,920
So delta minus means it's
a little bit negative.

812
00:40:14,920 --> 00:40:18,100
And we've got charge neutrality,
so if the fluorine

813
00:40:18,100 --> 00:40:21,170
end is a little bit negative
then the hydrogen end has to

814
00:40:21,170 --> 00:40:24,150
be a little bit positive, which
means this thing has a

815
00:40:24,150 --> 00:40:27,320
net dipole moment.

816
00:40:27,320 --> 00:40:29,070
It's a dipole.

817
00:40:29,070 --> 00:40:31,430
And the arrow points to
the negative end.

818
00:40:31,430 --> 00:40:34,730
One way to think about it is I
put a little slash there and

819
00:40:34,730 --> 00:40:36,700
that starts to look a little
bit like a plus sign.

820
00:40:36,700 --> 00:40:39,160
You can come up with your
own way to remember it.

821
00:40:39,160 --> 00:40:41,590
So it's got a little bit
of a dipole moment.

822
00:40:41,590 --> 00:40:45,620
And people depict dipoles
usually as ovals, and they'll

823
00:40:45,620 --> 00:40:47,800
put a minus end and
a plus end.

824
00:40:47,800 --> 00:40:51,390
So it's net neutral but the
charge is not uniformly

825
00:40:51,390 --> 00:40:52,750
distributed.

826
00:40:52,750 --> 00:40:53,980
OK?

827
00:40:53,980 --> 00:40:57,410
So our sharing meter in this
case is going to show

828
00:40:57,410 --> 00:41:00,110
something to the right.

829
00:41:00,110 --> 00:41:03,900
We've got electrons that are
unequally shared, and that

830
00:41:03,900 --> 00:41:05,580
moves over to the right.

831
00:41:05,580 --> 00:41:10,890
And, you know, the dipoles have
interesting properties.

832
00:41:10,890 --> 00:41:12,530
Oh, there's a plot of
electronegativity

833
00:41:12,530 --> 00:41:15,620
3-bar in the bar plot.

834
00:41:15,620 --> 00:41:17,590
And actually this is
an interesting one.

835
00:41:17,590 --> 00:41:20,310
Just parenthetically, you
see hydrogen here?

836
00:41:20,310 --> 00:41:22,340
Hydrogen's weird.

837
00:41:22,340 --> 00:41:25,070
They put it in the periodic
table above lithium but it's

838
00:41:25,070 --> 00:41:27,040
not an alkaline metal.

839
00:41:27,040 --> 00:41:29,190
And you can see it just
doesn't belong there.

840
00:41:29,190 --> 00:41:31,690
And there's a lot of
conversation about putting it

841
00:41:31,690 --> 00:41:36,360
maybe somewhere centered above
the p block elements, because

842
00:41:36,360 --> 00:41:39,020
it certainly doesn't belong
next to helium.

843
00:41:39,020 --> 00:41:43,470
But it probably doesn't belong
above lithium either.

844
00:41:43,470 --> 00:41:45,030
Anyway, I thought that
was very interesting.

845
00:41:45,030 --> 00:41:47,910
I can tell from the
response of the

846
00:41:47,910 --> 00:41:50,190
class, why does he care?

847
00:41:50,190 --> 00:41:50,490
[LAUGHTER]

848
00:41:50,490 --> 00:41:51,370
PROFESSOR: All right.

849
00:41:51,370 --> 00:41:53,370
Now this is really--

850
00:41:53,370 --> 00:41:54,360
I'm going to use an
adverb here--

851
00:41:54,360 --> 00:41:55,846
this is really important.

852
00:41:55,846 --> 00:41:56,590
All right?

853
00:41:56,590 --> 00:42:01,410
So here's HCl, is a cousin of
HF, and you see in the upper

854
00:42:01,410 --> 00:42:05,430
frame it's just a bunch of HCl
molecules just bopping around

855
00:42:05,430 --> 00:42:06,990
any which way.

856
00:42:06,990 --> 00:42:09,850
So there's the delta plus
and the delta minus.

857
00:42:09,850 --> 00:42:13,380
Now if you take these dipoles
and you put them in an

858
00:42:13,380 --> 00:42:18,340
electric field they will align
themselves, and the positive

859
00:42:18,340 --> 00:42:22,275
ends will face the negative
plate and the negative ends

860
00:42:22,275 --> 00:42:23,800
will face the positive plate.

861
00:42:23,800 --> 00:42:28,960
And there's energy stored when
the random orientation goes

862
00:42:28,960 --> 00:42:31,090
into an ordered orientation.

863
00:42:31,090 --> 00:42:33,880
This is the principle
behind a capacitor.

864
00:42:33,880 --> 00:42:35,800
A capacitor is nothing
more than a whole

865
00:42:35,800 --> 00:42:37,790
bunch of aligned dipoles.

866
00:42:37,790 --> 00:42:41,460
So if you want to invent a
supercapacitor that we can use

867
00:42:41,460 --> 00:42:44,870
on a car to extend the range
of the automobile so we can

868
00:42:44,870 --> 00:42:47,960
reduce our dependence on
imported petroleum, you're

869
00:42:47,960 --> 00:42:49,650
going to look for molecules
that have a

870
00:42:49,650 --> 00:42:51,970
honking big dipole moment.

871
00:42:51,970 --> 00:42:57,770
That way you get more energy
per unit electric field.

872
00:42:57,770 --> 00:43:02,460
So, again, a simple idea that
tells me how to go and invent.

873
00:43:02,460 --> 00:43:04,830
I can go back to my office and
go and invent something right

874
00:43:04,830 --> 00:43:07,320
now just based on
this lecture 9.

875
00:43:07,320 --> 00:43:08,670
[LAUGHTER]

876
00:43:08,670 --> 00:43:09,930
PROFESSOR: See, you
go and invent.

877
00:43:09,930 --> 00:43:12,050
You start the company,
you make the billion.

878
00:43:12,050 --> 00:43:14,670
Remember good old Professor
Sadoway at MIT, and

879
00:43:14,670 --> 00:43:17,780
established the fellowship
for students, and so.

880
00:43:17,780 --> 00:43:18,260
All right.

881
00:43:18,260 --> 00:43:21,280
But you have to know what
a dipole moment is.

882
00:43:21,280 --> 00:43:23,320
Got to know what a
dipole moment is.

883
00:43:23,320 --> 00:43:23,640
OK.

884
00:43:23,640 --> 00:43:24,860
So there's the dipole moment.

885
00:43:24,860 --> 00:43:29,100
And then lastly I'm going
to put sodium chloride.

886
00:43:29,100 --> 00:43:30,820
So what's sodium chloride
look like?

887
00:43:30,820 --> 00:43:33,250
Well it's Na plus
and Cl minus.

888
00:43:33,250 --> 00:43:36,330
So the electron has transferred
completely.

889
00:43:36,330 --> 00:43:38,670
So this isn't even
sharing at all.

890
00:43:38,670 --> 00:43:41,020
So this is really
bury the needle.

891
00:43:41,020 --> 00:43:42,270
This is not sharing.

892
00:43:45,230 --> 00:43:49,570
In this instance the sodium
doesn't even get visitation

893
00:43:49,570 --> 00:43:50,555
rights to the electron.

894
00:43:50,555 --> 00:43:53,050
The electron's gone.

895
00:43:53,050 --> 00:43:56,990
Whereas here hydrogen gets to
see the electron on Saturdays

896
00:43:56,990 --> 00:43:58,190
kind of thing.

897
00:43:58,190 --> 00:44:03,000
Depends what kind of lawyer
fluorine had.

898
00:44:03,000 --> 00:44:04,990
That's what it all
boils down to.

899
00:44:04,990 --> 00:44:07,390
All right.

900
00:44:07,390 --> 00:44:09,690
This is the same thing that
I just showed you.

901
00:44:09,690 --> 00:44:12,380
But you see, the textbook
gives you, as the name

902
00:44:12,380 --> 00:44:13,920
implies, dense text.

903
00:44:13,920 --> 00:44:15,320
I gave you the sharing meter.

904
00:44:15,320 --> 00:44:19,380
The sharing meter is far
more expositive.

905
00:44:19,380 --> 00:44:19,740
All right.

906
00:44:19,740 --> 00:44:22,990
And then, finally, the percent
ionic character is given by

907
00:44:22,990 --> 00:44:24,050
this formula here.

908
00:44:24,050 --> 00:44:26,810
So this is 1 minus
the exponential.

909
00:44:26,810 --> 00:44:34,840
So the exp term, this
exponential of-- what is it--

910
00:44:34,840 --> 00:44:38,670
minus 1/4 times the
difference in

911
00:44:38,670 --> 00:44:41,220
electronegativities squared.

912
00:44:41,220 --> 00:44:47,380
This notation means e base
natural logarithms, minus 1/4,

913
00:44:47,380 --> 00:44:47,960
blah, blah, blah.

914
00:44:47,960 --> 00:44:49,250
That's what this thing is.

915
00:44:49,250 --> 00:44:52,080
So if you plug in, multiply by
100% you get something that

916
00:44:52,080 --> 00:44:53,680
goes from 0 to 100.

917
00:44:53,680 --> 00:44:59,540
So obviously when delta
chi is 0 you get 0%.

918
00:44:59,540 --> 00:45:02,150
e to the 0 is 1, 1 minus
1 is 0, and so you

919
00:45:02,150 --> 00:45:04,030
have no ionic character.

920
00:45:04,030 --> 00:45:08,130
And so if you plug in
the numbers for HF--

921
00:45:08,130 --> 00:45:13,620
so you're going to take this
difference here, square it--

922
00:45:13,620 --> 00:45:17,210
it ends up giving you 1.8, which
gives you a value of

923
00:45:17,210 --> 00:45:20,040
about 56% ionic character.

924
00:45:20,040 --> 00:45:24,490
So it's as though the electron
is sort of half transferred.

925
00:45:24,490 --> 00:45:27,760
But you might also look at
it from this perspective.

926
00:45:27,760 --> 00:45:30,110
If you take 344--

927
00:45:30,110 --> 00:45:37,120
because this is the partial
ionic character, which is the

928
00:45:37,120 --> 00:45:40,260
energy of electron displacement
over the total

929
00:45:40,260 --> 00:45:42,280
energy in the calculation--

930
00:45:42,280 --> 00:45:45,000
that turns out to be 57%.

931
00:45:45,000 --> 00:45:46,610
So this stuff makes sense.

932
00:45:46,610 --> 00:45:49,390
There's a sensible metric
here at work.

933
00:45:49,390 --> 00:45:53,720
And so this is what Linus
Pauling got his Nobel Prize

934
00:45:53,720 --> 00:45:57,190
for, and it's the description
of polar covalency.

935
00:46:00,470 --> 00:46:02,850
And polar covalency is operative
when you have

936
00:46:02,850 --> 00:46:06,560
heteronuclear bonds, because
the two different elements

937
00:46:06,560 --> 00:46:09,920
don't share the electron
equally.

938
00:46:09,920 --> 00:46:12,525
And the Pauling formula allows
you to calculate that.

939
00:46:12,525 --> 00:46:17,900
And his formative book was
written in 1937, called The

940
00:46:17,900 --> 00:46:20,060
Nature of the Chemical Bond.

941
00:46:20,060 --> 00:46:20,870
OK.

942
00:46:20,870 --> 00:46:25,100
So turning to the last five
minutes, I want to bring to

943
00:46:25,100 --> 00:46:28,110
your attention some covalent
molecules.

944
00:46:28,110 --> 00:46:31,110
Today we're going to
talk about Freon.

945
00:46:31,110 --> 00:46:35,270
Freon was an invention, it was
a designer chemical, invented

946
00:46:35,270 --> 00:46:36,850
by Thomas Midgley.

947
00:46:36,850 --> 00:46:37,560
This is me.

948
00:46:37,560 --> 00:46:40,700
I named him "sp3." That's
his nickname.

949
00:46:40,700 --> 00:46:44,660
Thomas sp3, for the hybridized
orbital.

950
00:46:44,660 --> 00:46:47,810
So he was working at the
Dayton engineering

951
00:46:47,810 --> 00:46:51,130
laboratories in Dayton, which
was owned by General Motors,

952
00:46:51,130 --> 00:46:54,530
and he was working in the 20s
at a time when there were no

953
00:46:54,530 --> 00:46:57,160
refrigerators in American
kitchens.

954
00:46:57,160 --> 00:47:00,080
The only refrigerants that were
used were either toxic or

955
00:47:00,080 --> 00:47:02,160
flammable, things like ammonia,
methyl chloride,

956
00:47:02,160 --> 00:47:03,620
sulfur dioxide.

957
00:47:03,620 --> 00:47:05,390
And you read about horrible
accidents.

958
00:47:05,390 --> 00:47:09,740
People making ice cream at some
plant and the compressor

959
00:47:09,740 --> 00:47:12,170
blows up and two or three
people are killed.

960
00:47:12,170 --> 00:47:15,820
So it was deemed unsafe in
the American kitchen.

961
00:47:15,820 --> 00:47:19,660
In the 20s Midgley discovered
this molecule, which looks

962
00:47:19,660 --> 00:47:23,110
just like methane only we've
replace the hydrogens with two

963
00:47:23,110 --> 00:47:25,100
chlorines and two fluorines.

964
00:47:25,100 --> 00:47:28,440
So this is called
dichlorodifluoromethane and

965
00:47:28,440 --> 00:47:32,050
it's a chlorofluorocarbon,
a CFC.

966
00:47:32,050 --> 00:47:35,480
And this was fantastic stuff.

967
00:47:35,480 --> 00:47:41,170
It it was colorless, odorless,
tasteless, non-toxic.

968
00:47:41,170 --> 00:47:44,010
It was not just used as a
refrigerant, it was used in

969
00:47:44,010 --> 00:47:44,720
propellant.

970
00:47:44,720 --> 00:47:49,160
When I was your age all
of the sprays--

971
00:47:49,160 --> 00:47:53,400
whether it was hair spray,
shaving cream, any aerosol--

972
00:47:53,400 --> 00:47:56,255
was propelled by Freon-12.

973
00:47:56,255 --> 00:47:58,910
It was fantastic stuff.

974
00:47:58,910 --> 00:48:02,450
Well, it turns out that in
the upper atmosphere--

975
00:48:02,450 --> 00:48:03,710
you know, you go pss pss pss.

976
00:48:03,710 --> 00:48:06,610
You got people all over the
world doing this, eventually

977
00:48:06,610 --> 00:48:08,760
this stuff starts
floating away.

978
00:48:08,760 --> 00:48:11,130
And what turns out in the upper
atmosphere where we

979
00:48:11,130 --> 00:48:14,080
don't have shielding
from ultraviolet--

980
00:48:14,080 --> 00:48:16,190
you know how to do this
calculation, because you can

981
00:48:16,190 --> 00:48:18,070
look up the energy.

982
00:48:18,070 --> 00:48:20,040
And, in fact, it's part of your
homework, where you look

983
00:48:20,040 --> 00:48:24,070
at the energy differences and
the electronegativity

984
00:48:24,070 --> 00:48:26,960
differences, you can compute the
wavelength of light that's

985
00:48:26,960 --> 00:48:29,470
capable of breaking the
carbon-chlorine bond.

986
00:48:29,470 --> 00:48:31,950
And it turns out to be
in the ultraviolet.

987
00:48:31,950 --> 00:48:35,680
Once the chlorine is broken you
have a chlorine radical,

988
00:48:35,680 --> 00:48:39,330
and that chlorine radical goes
over here and attacks ozone.

989
00:48:39,330 --> 00:48:40,340
[CELL PHONE RINGING]

990
00:48:40,340 --> 00:48:41,280
PROFESSOR: Cell phone--

991
00:48:41,280 --> 00:48:42,530
out.

992
00:48:45,630 --> 00:48:48,879
Just get up and leave
out of courtesy.

993
00:48:48,879 --> 00:48:50,129
[CELL PHONE STILL RINGING]

994
00:48:58,360 --> 00:48:59,610
[LAUGHTER]

995
00:49:04,870 --> 00:49:06,120
PROFESSOR: Hello?

996
00:49:08,490 --> 00:49:13,440
The first year I was teaching
3.091 there was a Nobel Prize

997
00:49:13,440 --> 00:49:17,570
awarded to Mario Molina, who was
a faculty member here in

998
00:49:17,570 --> 00:49:21,720
Earth and Planetary Sciences who
had worked years earlier

999
00:49:21,720 --> 00:49:26,230
at University of California,
Irvine, and had speculated on

1000
00:49:26,230 --> 00:49:31,540
the mechanism by which ozone
depletion occurs and linked it

1001
00:49:31,540 --> 00:49:34,790
to rising levels of CFCs.

1002
00:49:34,790 --> 00:49:36,040
Initially--

1003
00:49:36,040 --> 00:49:38,180
that's why it says
a vindication--

1004
00:49:38,180 --> 00:49:40,410
people pooh poohed it,
said it was crazy.

1005
00:49:40,410 --> 00:49:43,100
There wasn't enough of this pss
pss to cause any trouble.

1006
00:49:43,100 --> 00:49:45,870
But then later with the NASA
program they started taking a

1007
00:49:45,870 --> 00:49:48,780
lot of images and they could
track ozone levels in the

1008
00:49:48,780 --> 00:49:51,850
atmosphere and start seeing
that not only was ozone

1009
00:49:51,850 --> 00:49:54,240
changing but there were actually
pockets where ozone

1010
00:49:54,240 --> 00:49:57,150
was being depleted at an
accelerating rate-- because

1011
00:49:57,150 --> 00:50:00,400
obviously the atmosphere isn't
constant composition and

1012
00:50:00,400 --> 00:50:01,200
constant temperature.

1013
00:50:01,200 --> 00:50:01,680
Duh.

1014
00:50:01,680 --> 00:50:03,310
So anyways, yeah.

1015
00:50:03,310 --> 00:50:04,270
There he is.

1016
00:50:04,270 --> 00:50:06,320
And this was the paper that was

1017
00:50:06,320 --> 00:50:09,670
published in 1974 in Nature.

1018
00:50:09,670 --> 00:50:13,410
And this was done before
computers.

1019
00:50:13,410 --> 00:50:18,560
The PC wasn't invented
and commercialized

1020
00:50:18,560 --> 00:50:19,630
until the early 80s.

1021
00:50:19,630 --> 00:50:21,990
So this was typeset, and the
person who typeset it

1022
00:50:21,990 --> 00:50:25,080
obviously didn't take 3.091
because instead of "atom"

1023
00:50:25,080 --> 00:50:30,420
hyphen "catalysed" we have
"atomc-atalysed." But even

1024
00:50:30,420 --> 00:50:35,130
ignoring the spelling error in
a Nobel Prize winning paper--

1025
00:50:35,130 --> 00:50:36,890
[LAUGHTER]

1026
00:50:36,890 --> 00:50:39,340
PROFESSOR: --the Nobel committee
overlooked this.

1027
00:50:39,340 --> 00:50:41,040
Yeah.

1028
00:50:41,040 --> 00:50:42,000
So there it is.

1029
00:50:42,000 --> 00:50:44,900
And then they went to
HFCs and so on.

1030
00:50:44,900 --> 00:50:48,300
There's a lot of activity
in this.

1031
00:50:48,300 --> 00:50:51,800
And what happened is when we
changed from CFCs to HFCs we

1032
00:50:51,800 --> 00:50:53,920
had to change the design
of the compressors.

1033
00:50:53,920 --> 00:50:56,050
And what happened was
everything got

1034
00:50:56,050 --> 00:50:57,760
much, much more efficient.

1035
00:50:57,760 --> 00:51:01,420
So this was an example of
necessity for a change that

1036
00:51:01,420 --> 00:51:03,320
was driven by concern
for the environment.

1037
00:51:03,320 --> 00:51:05,820
Instead of putting people out
of work and killing an

1038
00:51:05,820 --> 00:51:08,942
industry, gave us much more
efficient refrigeration.

1039
00:51:08,942 --> 00:51:10,830
And the last thing I'll
show you is this

1040
00:51:10,830 --> 00:51:11,620
to draw your attention.

1041
00:51:11,620 --> 00:51:13,280
This was in your textbook.

1042
00:51:13,280 --> 00:51:16,770
This is the cap at the top of
the Washington Monument.

1043
00:51:16,770 --> 00:51:19,080
The Washington Monument was
built to celebrate the

1044
00:51:19,080 --> 00:51:21,530
American centennial, 1876.

1045
00:51:21,530 --> 00:51:23,560
They finished it in 1884.

1046
00:51:23,560 --> 00:51:26,780
And this is 100 ounces of
aluminum, because aluminum was

1047
00:51:26,780 --> 00:51:27,970
a precious metal.

1048
00:51:27,970 --> 00:51:30,100
It was priced higher
than silver.

1049
00:51:30,100 --> 00:51:31,090
1884.

1050
00:51:31,090 --> 00:51:34,490
Two years later Charles Martin
Hall and Paul Heroult invent

1051
00:51:34,490 --> 00:51:37,140
an electrochemical process
that drives the price of

1052
00:51:37,140 --> 00:51:40,430
aluminum down to the point
that we make beer cans--

1053
00:51:40,430 --> 00:51:42,426
I mean soda cans--
out of it today.

1054
00:51:42,426 --> 00:51:42,890
[LAUGHTER]

1055
00:51:42,890 --> 00:51:47,580
PROFESSOR: And a good example of
how chemical innovation can

1056
00:51:47,580 --> 00:51:48,940
lead to superior products.

1057
00:51:48,940 --> 00:51:50,680
I'll see you on Wednesday.