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PROFESSOR: OK, a couple
of announcements.

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00:00:23,500 --> 00:00:27,500
Tomorrow will be the Periodic
Table test. I provide the

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00:00:27,500 --> 00:00:29,430
numbers, you provide
the letters.

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00:00:29,430 --> 00:00:32,430
Friday, the contest
ends, 5:00 pm.

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00:00:32,430 --> 00:00:36,440
Get the submissions to me,
either over the internet or if

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00:00:36,440 --> 00:00:39,960
you have something majestic,
you have to cart it in with

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00:00:39,960 --> 00:00:41,080
the riggers.

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00:00:41,080 --> 00:00:42,330
Get it to my office.

16
00:00:44,680 --> 00:00:48,600
Last day, we had a lesson
on the Aufbau principle.

17
00:00:48,600 --> 00:00:52,080
And the Aufbau principle
gave us the--

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00:00:52,080 --> 00:00:54,280
There's still too
much talking.

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00:00:54,280 --> 00:00:55,270
Still too much talking.

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00:00:55,270 --> 00:00:59,830
I will tolerate zero,
zero talking.

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00:00:59,830 --> 00:01:03,990
When you walk through that door,
I'm assuming it's an act

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00:01:03,990 --> 00:01:05,790
of free will.

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00:01:05,790 --> 00:01:09,080
And when you walk through
that door, we

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00:01:09,080 --> 00:01:12,520
enter into a contract.

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00:01:12,520 --> 00:01:15,440
The contract goes something like
this: You have certain

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00:01:15,440 --> 00:01:17,250
expectations of me.

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00:01:17,250 --> 00:01:20,040
You expect me to come
to class prepared.

28
00:01:20,040 --> 00:01:24,080
You expect me to treat you
with respect, not to use

29
00:01:24,080 --> 00:01:27,900
vulgar language, not to say
things that are insulting,

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00:01:27,900 --> 00:01:29,140
insensitive.

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00:01:29,140 --> 00:01:32,620
And I have certain expectations
of you.

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00:01:32,620 --> 00:01:34,670
I expect you to come
to class prepared.

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00:01:34,670 --> 00:01:36,480
I expect that you've
done the reading.

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00:01:36,480 --> 00:01:38,820
And I expect that you're
going to sit quietly.

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00:01:38,820 --> 00:01:40,990
Absolutely quietly.

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00:01:40,990 --> 00:01:44,510
Because it's my duty to preserve
a fertile learning

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00:01:44,510 --> 00:01:45,120
environment.

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00:01:45,120 --> 00:01:48,110
If you don't want to learn,
I don't care.

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00:01:48,110 --> 00:01:48,670
I don't care.

40
00:01:48,670 --> 00:01:52,570
But if you impair the ability
of your neighbor to learn, I

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00:01:52,570 --> 00:01:55,830
will take action.

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00:01:55,830 --> 00:01:58,120
It's very simple.

43
00:01:58,120 --> 00:01:59,370
Very simple.

44
00:02:04,520 --> 00:02:05,410
Where was I?

45
00:02:05,410 --> 00:02:12,110
I think I was talking about
the Aufbau principle.

46
00:02:12,110 --> 00:02:15,050
And the Aufbau principle tells
us what the filling sequence

47
00:02:15,050 --> 00:02:19,710
of electrons is in a
multielectron atom, which

48
00:02:19,710 --> 00:02:23,420
ultimately we rationalized in
terms of the four quantum

49
00:02:23,420 --> 00:02:28,540
numbers and the filling in
ascending order of energy.

50
00:02:28,540 --> 00:02:31,850
And then we tried to understand
what's behind the

51
00:02:31,850 --> 00:02:33,040
Aufbau principle.

52
00:02:33,040 --> 00:02:37,280
And lastly, we took a journey
through central Europe in the

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00:02:37,280 --> 00:02:39,130
1920s and 1930s.

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00:02:39,130 --> 00:02:41,330
And we met de Broglie,
who gave us the

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00:02:41,330 --> 00:02:43,040
concept of matter waves.

56
00:02:43,040 --> 00:02:46,630
If matter has wave-like
properties, the wavelength

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00:02:46,630 --> 00:02:49,870
would be given by the ratio of
the Planck constant to the

58
00:02:49,870 --> 00:02:52,160
Newtonian momentum.

59
00:02:52,160 --> 00:02:55,660
Heisenberg taught us that we
can't deal with precision down

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00:02:55,660 --> 00:02:58,150
to atomic dimensions, and
there's a certain degree of

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00:02:58,150 --> 00:03:00,420
uncertainty or indeterminacy.

62
00:03:00,420 --> 00:03:03,650
And it's given by this
relationship here.

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00:03:03,650 --> 00:03:06,540
And then, lastly, we saw
Schroedinger, who wrote the

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00:03:06,540 --> 00:03:10,440
wave equation saying that if we
have wave-like properties

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00:03:10,440 --> 00:03:15,210
then let's model the atom as
a wave, and he gave us this

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equation here, which is
essentially the equation of

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00:03:18,880 --> 00:03:21,172
the simple harmonic
oscillator.

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00:03:21,172 --> 00:03:24,370
The simple harmonic oscillator,
which allows you

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00:03:24,370 --> 00:03:27,630
to tell us what's going on
with the plucked string.

70
00:03:27,630 --> 00:03:31,670
And it's essentially just this,
the double derivative x

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00:03:31,670 --> 00:03:34,670
double dot plus kx goes
to some function.

72
00:03:34,670 --> 00:03:38,610
And the solution of that is a
bunch of sines and cosines.

73
00:03:38,610 --> 00:03:41,910
And that's this equation, but
dressed up for night-time.

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00:03:41,910 --> 00:03:45,040
It's a little bit more
sophisticated and gives us the

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00:03:45,040 --> 00:03:47,890
three dimensions, eigenfunctions
that

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00:03:47,890 --> 00:03:51,650
ultimately, we can take and
generate the pretty pictures

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00:03:51,650 --> 00:03:53,020
of the orbitals.

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00:03:53,020 --> 00:03:57,020
So, today what I want to do is
go a little more deeply into

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properties.

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00:03:57,750 --> 00:04:03,180
And if you learn nothing else
from me in 3.091, this is what

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00:04:03,180 --> 00:04:04,750
I want you to retain.

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00:04:04,750 --> 00:04:09,170
It's that electronic structure
dictates properties.

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00:04:09,170 --> 00:04:12,290
Electronic structure dictates
properties.

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00:04:12,290 --> 00:04:14,570
You know this on
your death bed.

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00:04:14,570 --> 00:04:15,730
It's that simple.

86
00:04:15,730 --> 00:04:17,730
Electronic structure dictates
properties.

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00:04:17,730 --> 00:04:21,640
Then we're going to take 14
weeks and describe electronic

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00:04:21,640 --> 00:04:23,110
structure, right?

89
00:04:23,110 --> 00:04:24,500
But that's it.

90
00:04:24,500 --> 00:04:25,940
That's it.

91
00:04:25,940 --> 00:04:28,290
So I want to go back to this.

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00:04:28,290 --> 00:04:32,380
We got into this energy, Aufbau
principle because we

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00:04:32,380 --> 00:04:35,060
saw on the Periodic Table
there's some problems here.

94
00:04:35,060 --> 00:04:38,180
If we just go in terms of
quantum numbers, we see

95
00:04:38,180 --> 00:04:41,710
there's two elements in
n equals 1 shell.

96
00:04:41,710 --> 00:04:44,300
There's 8 elements in
n equals 2 shell.

97
00:04:44,300 --> 00:04:47,440
And, if we just kept going,
we would expect to find 18

98
00:04:47,440 --> 00:04:50,850
elements in n equals 3 shell,
but instead we found only 8.

99
00:04:50,850 --> 00:04:53,130
And that's where the
journey began.

100
00:04:53,130 --> 00:04:58,060
Well, there's a way to sort of
help remember what the filling

101
00:04:58,060 --> 00:04:59,260
sequence is.

102
00:04:59,260 --> 00:05:06,560
And that rule is encapsulated by
the n plus l relationship.

103
00:05:06,560 --> 00:05:14,870
So for equivalent, n plus l
values, for equivalent n plus

104
00:05:14,870 --> 00:05:21,900
l values, you fill
in ascending n.

105
00:05:21,900 --> 00:05:26,730
Fill in ascending n.

106
00:05:26,730 --> 00:05:28,580
So let's take a look.

107
00:05:28,580 --> 00:05:31,630
So, here's Aufbau.

108
00:05:31,630 --> 00:05:32,760
n plus l rule.

109
00:05:32,760 --> 00:05:35,240
Here's an example where
we have 3d.

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00:05:35,240 --> 00:05:38,020
d means l equals 2.

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00:05:38,020 --> 00:05:40,860
p means l equals 1. s
means l equals 0.

112
00:05:40,860 --> 00:05:42,470
So 5 plus 0 is 5.

113
00:05:42,470 --> 00:05:43,610
4 plus 1 is 5.

114
00:05:43,610 --> 00:05:45,090
3 plus 2 is 5.

115
00:05:45,090 --> 00:05:46,010
So what do I do?

116
00:05:46,010 --> 00:05:52,200
Well, it's saying go in order
of ascending n plus l.

117
00:05:52,200 --> 00:05:56,740
And so, you can snake your way
through this n plus l, and get

118
00:05:56,740 --> 00:05:59,500
the filling sequence for the
whole Periodic Table.

119
00:05:59,500 --> 00:06:00,500
So let's take a look.

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00:06:00,500 --> 00:06:01,690
We'll start with 1s.

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00:06:01,690 --> 00:06:02,980
Well, that's easy.

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00:06:02,980 --> 00:06:04,610
That's hydrogen and helium.

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00:06:04,610 --> 00:06:07,650
Then we go next to 2s.

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00:06:07,650 --> 00:06:09,330
There's lithium and beryllium.

125
00:06:09,330 --> 00:06:10,870
And then next goes 2p.

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00:06:10,870 --> 00:06:14,860
I mean, nobody's going to try
to stick 3s ahead of 2p.

127
00:06:14,860 --> 00:06:16,700
That's pretty straightforward.

128
00:06:16,700 --> 00:06:19,020
2p, and that gets us all
the way over to neon.

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00:06:19,020 --> 00:06:21,350
And then we wrap around
over here to 3s.

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00:06:21,350 --> 00:06:24,370
That gets us from sodium
to magnesium.

131
00:06:24,370 --> 00:06:28,950
And then we jump to
3p, gets us from

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00:06:28,950 --> 00:06:30,680
aluminum over to argon.

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00:06:30,680 --> 00:06:35,850
And then, we don't go to 3d,
because 3p is 3 plus 1 is 4.

134
00:06:35,850 --> 00:06:38,160
4s is 4 plus 0 is 4.

135
00:06:38,160 --> 00:06:41,950
So it says we fill
in ascending n.

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00:06:41,950 --> 00:06:43,550
So we go to 4s next.

137
00:06:43,550 --> 00:06:45,990
And that gets us potassium
and calcium.

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00:06:45,990 --> 00:06:47,550
Then comes 3d.

139
00:06:47,550 --> 00:06:51,640
So that gets us scandium over
to zinc. Then comes 4p,

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00:06:51,640 --> 00:06:53,360
gallium to krypton.

141
00:06:53,360 --> 00:06:54,520
5s.

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00:06:54,520 --> 00:06:56,530
I'm having fun here, I'm
going to keep going.

143
00:06:56,530 --> 00:06:59,230
5s gets us over to strontium.

144
00:06:59,230 --> 00:07:01,270
Next comes 4d.

145
00:07:01,270 --> 00:07:03,430
Gets us all the way
over to cadmium.

146
00:07:03,430 --> 00:07:06,070
5p gets us to xenon.

147
00:07:06,070 --> 00:07:10,410
Now it's turn 6s
cesium, barium.

148
00:07:10,410 --> 00:07:11,295
4f.

149
00:07:11,295 --> 00:07:12,780
We've got to jump down here.

150
00:07:12,780 --> 00:07:15,130
Finally we're going to
get to the f shell.

151
00:07:15,130 --> 00:07:16,280
See?

152
00:07:16,280 --> 00:07:17,670
The 4f is in here.

153
00:07:17,670 --> 00:07:22,430
Now, this is an item
of convenience.

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00:07:22,430 --> 00:07:27,590
Strictly speaking, lanthanum
is 57, cerium is 58.

155
00:07:27,590 --> 00:07:30,730
So all of these elements
belong in here.

156
00:07:30,730 --> 00:07:33,400
But if we were to put this to
scale, the Periodic Table

157
00:07:33,400 --> 00:07:34,245
would be way over here.

158
00:07:34,245 --> 00:07:35,940
It wouldn't fit on a page.

159
00:07:35,940 --> 00:07:38,810
So people, over time, have
gotten used to just

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00:07:38,810 --> 00:07:39,750
putting it down here.

161
00:07:39,750 --> 00:07:42,680
But these elements are
in the middle here.

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00:07:42,680 --> 00:07:48,670
So you go 4d, 5p, 6s, then 4f.

163
00:07:48,670 --> 00:07:51,180
Finally we get over to the edge
here, to erbium, and then

164
00:07:51,180 --> 00:07:55,110
we jump over here to hafnium,
5d, over to

165
00:07:55,110 --> 00:07:59,410
mercury, 6p, et cetera.

166
00:07:59,410 --> 00:08:02,880
So the n plus l rule gives you
the filling sequence in

167
00:08:02,880 --> 00:08:04,440
ascending order.

168
00:08:04,440 --> 00:08:05,500
That's good.

169
00:08:05,500 --> 00:08:08,910
So we've got a nice compact
way of grabbing this e

170
00:08:08,910 --> 00:08:10,730
goes to n plus l.

171
00:08:10,730 --> 00:08:12,850
Now let's look at
some properties.

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00:08:12,850 --> 00:08:14,570
We said it's a table of
the elements, but

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00:08:14,570 --> 00:08:15,730
it's a periodic table.

174
00:08:15,730 --> 00:08:18,430
So let's see what this
periodicity looks like.

175
00:08:18,430 --> 00:08:21,250
Now, here's a variation of
first ionization energy.

176
00:08:21,250 --> 00:08:25,550
So here it is in kilojoules per
mole, and here's my pet

177
00:08:25,550 --> 00:08:27,700
unit, the electron
volt per atom.

178
00:08:27,700 --> 00:08:33,500
Because here's hydrogen, of 1.6
megajoules per mole, but I

179
00:08:33,500 --> 00:08:36,920
like the 13.6 electron volts
because I can remember 13.6.

180
00:08:36,920 --> 00:08:41,040
I can't remember 1312
megajoules per mole,

181
00:08:41,040 --> 00:08:43,130
kilojoules per mole,
whatever it is.

182
00:08:43,130 --> 00:08:44,920
13.6 electronic volts.

183
00:08:44,920 --> 00:08:46,930
So here's hydrogen,
there's helium.

184
00:08:46,930 --> 00:08:49,760
And then we drop down to
lithium, and then we move

185
00:08:49,760 --> 00:08:52,170
across up to neon, and so on.

186
00:08:52,170 --> 00:08:55,950
So you can see 1s, we
filled the 1s shell.

187
00:08:55,950 --> 00:08:58,920
Now, we're going to go
to shell n equals 2.

188
00:08:58,920 --> 00:09:00,720
Here's 2s.

189
00:09:00,720 --> 00:09:02,250
And then we go to 2p.

190
00:09:02,250 --> 00:09:05,770
And you can even see, look,
boron, carbon, oxygen, the

191
00:09:05,770 --> 00:09:10,820
ascending value of the first
ionization energy.

192
00:09:10,820 --> 00:09:12,780
And then there's a little
jog here at oxygen.

193
00:09:12,780 --> 00:09:15,140
Because here we've got
the three unpaired

194
00:09:15,140 --> 00:09:17,150
electrons for nitrogen.

195
00:09:17,150 --> 00:09:19,280
And then there's the oxygen
starts to pair, and we

196
00:09:19,280 --> 00:09:21,130
continue to neon, and so on.

197
00:09:21,130 --> 00:09:23,790
3s, 3p, 4s.

198
00:09:23,790 --> 00:09:26,510
So you can see the relationship
between that

199
00:09:26,510 --> 00:09:30,650
property, and the place,
the element, in

200
00:09:30,650 --> 00:09:32,120
the Periodic Table.

201
00:09:32,120 --> 00:09:34,050
So how do we measure
these values?

202
00:09:34,050 --> 00:09:38,690
Well, I thought it might be a
good opportunity to look at,

203
00:09:38,690 --> 00:09:41,820
revisit the whole question
of gas dynamics, and also

204
00:09:41,820 --> 00:09:44,110
understand the measurement.

205
00:09:44,110 --> 00:09:51,690
Measurement of ionization
energies.

206
00:09:51,690 --> 00:09:55,890
And the technique that's used
is called photoelectron

207
00:09:55,890 --> 00:09:57,750
spectroscopy.

208
00:09:57,750 --> 00:09:59,172
Photoelectron spectroscopy.

209
00:10:03,460 --> 00:10:03,900
All right.

210
00:10:03,900 --> 00:10:06,520
And it's got a three-letter
initialization, PES.

211
00:10:09,420 --> 00:10:13,130
I want to show you a cartoon
of how this works.

212
00:10:13,130 --> 00:10:16,310
Actually, I took one --
this is taken from the

213
00:10:16,310 --> 00:10:17,630
text we used to use.

214
00:10:17,630 --> 00:10:20,250
I like the text that we're using
now better, but there

215
00:10:20,250 --> 00:10:22,100
are a few things in the other
text that were good.

216
00:10:22,100 --> 00:10:26,030
So, we've scanned these few
pages and posted them at the

217
00:10:26,030 --> 00:10:28,850
website if you want to go
through and read this stuff.

218
00:10:28,850 --> 00:10:31,650
So let's jump over to that.

219
00:10:31,650 --> 00:10:35,220
So here's an example of
a terrible drawing.

220
00:10:35,220 --> 00:10:38,040
I look at this and I haven't got
the faintest idea how this

221
00:10:38,040 --> 00:10:40,210
thing works.

222
00:10:40,210 --> 00:10:41,420
It's not the artist's fault.

223
00:10:41,420 --> 00:10:45,240
Somebody should have proofread
this thing.

224
00:10:45,240 --> 00:10:48,485
What they should be doing is
showing something like this.

225
00:10:48,485 --> 00:10:50,510
So, let's see what's going on.

226
00:10:50,510 --> 00:10:55,490
So what we're going to do is,
we're going to bombard a

227
00:10:55,490 --> 00:10:57,040
specimen right here.

228
00:10:57,040 --> 00:10:58,220
Which they never show.

229
00:10:58,220 --> 00:11:01,240
I suppose this atom beam is
supposed to be colliding with

230
00:11:01,240 --> 00:11:01,920
the photons.

231
00:11:01,920 --> 00:11:04,120
That would be quite
an apparatus to

232
00:11:04,120 --> 00:11:05,670
build, let me tell you.

233
00:11:05,670 --> 00:11:09,270
Instead, what you have is, you
have the apparatus sitting so

234
00:11:09,270 --> 00:11:13,260
that you've got the material in
the center of the chamber.

235
00:11:13,260 --> 00:11:15,270
And you irradiate
with photons.

236
00:11:15,270 --> 00:11:18,290
And the photons have very, very
high energy, and they

237
00:11:18,290 --> 00:11:21,220
dislodge electrons.

238
00:11:21,220 --> 00:11:25,030
When they dislodge electrons
that's the ionization event.

239
00:11:25,030 --> 00:11:27,930
And then what we do is, we
measure the velocity of those

240
00:11:27,930 --> 00:11:30,270
ejected electrons.

241
00:11:30,270 --> 00:11:32,750
And if we know the velocity,
we know their energy.

242
00:11:32,750 --> 00:11:35,720
We know the energy of the
incident energy and by

243
00:11:35,720 --> 00:11:40,813
difference, we calculate the
binding energy of the electron

244
00:11:40,813 --> 00:11:41,640
and the atom.

245
00:11:41,640 --> 00:11:42,630
So let's take a look.

246
00:11:42,630 --> 00:11:47,440
I'm going to draw a cartoon here
that gives you a sense of

247
00:11:47,440 --> 00:11:48,210
what's going on.

248
00:11:48,210 --> 00:11:53,980
So, here's the n equals 1
shell, or the k shell.

249
00:11:53,980 --> 00:11:57,940
And then I'm going to show,
say, a second shell here.

250
00:11:57,940 --> 00:12:00,120
One, two, three.

251
00:12:00,120 --> 00:12:03,690
So this is the l shell,
or n equals 2.

252
00:12:03,690 --> 00:12:05,510
And you might say, gee,
he's got that wrong.

253
00:12:05,510 --> 00:12:08,000
It's kind of simple,
and so on.

254
00:12:08,000 --> 00:12:13,190
It's as complex as it needs
to be for the explanation.

255
00:12:13,190 --> 00:12:16,770
Don't let accuracy
trump clarity.

256
00:12:16,770 --> 00:12:21,050
The real accurate picture here
is too detailed to convey what

257
00:12:21,050 --> 00:12:21,820
I'm trying to convey.

258
00:12:21,820 --> 00:12:24,610
So, this is the specimen,
right?

259
00:12:24,610 --> 00:12:28,000
Let's call this the specimen.

260
00:12:28,000 --> 00:12:32,520
And I'm going to irradiate the
specimen with some radiation

261
00:12:32,520 --> 00:12:34,150
of high energy.

262
00:12:34,150 --> 00:12:39,080
And so, I generally indicate the
photon by a squiggly line

263
00:12:39,080 --> 00:12:39,710
with an arrow.

264
00:12:39,710 --> 00:12:42,430
That's to indicate it's got
wave-like properties.

265
00:12:42,430 --> 00:12:45,580
And I'll usually write h nu next
to it, to indicate that

266
00:12:45,580 --> 00:12:48,270
it's a photon of energy h nu.

267
00:12:48,270 --> 00:12:51,640
So the photon comes in, and if
the energy is high enough, it

268
00:12:51,640 --> 00:12:54,626
will dislodge an electron.

269
00:12:54,626 --> 00:12:58,490
So this electron is dislodged.

270
00:13:02,270 --> 00:13:04,210
It's now ballistic.

271
00:13:04,210 --> 00:13:06,790
It's rendered ballistic.

272
00:13:06,790 --> 00:13:09,030
It's free.

273
00:13:09,030 --> 00:13:10,280
It's ionized.

274
00:13:13,230 --> 00:13:15,150
It's gone.

275
00:13:15,150 --> 00:13:16,540
It's gone.

276
00:13:16,540 --> 00:13:20,910
And what we're going to do is,
we're going to take an energy

277
00:13:20,910 --> 00:13:21,600
balance here.

278
00:13:21,600 --> 00:13:24,590
And at some point you may
come back and learn

279
00:13:24,590 --> 00:13:26,250
some quantum mechanics.

280
00:13:26,250 --> 00:13:29,780
This photon is annihilated
at this collision.

281
00:13:29,780 --> 00:13:32,950
So the photon doesn't act the
way an incident electron does

282
00:13:32,950 --> 00:13:34,520
where it loses some
of its energy.

283
00:13:34,520 --> 00:13:37,500
All of the energy is lost.
So this photon's gone.

284
00:13:37,500 --> 00:13:40,670
All of the energy is given
to this electron.

285
00:13:40,670 --> 00:13:42,340
So let's do an energy
balance here.

286
00:13:42,340 --> 00:13:47,840
So I can say that the energy
of the incident photon, the

287
00:13:47,840 --> 00:13:53,500
energy of the incident photon,
is now going to be given to

288
00:13:53,500 --> 00:13:54,620
the electron.

289
00:13:54,620 --> 00:13:57,820
It's now got some kind
of kinetic energy.

290
00:13:57,820 --> 00:14:03,520
E kinetic plus the energy
it took to pull

291
00:14:03,520 --> 00:14:06,250
it out of the atom.

292
00:14:06,250 --> 00:14:09,430
Plus E, let's call it binding.

293
00:14:09,430 --> 00:14:11,910
E binding

294
00:14:11,910 --> 00:14:14,540
And we know the incident energy
of the photon because

295
00:14:14,540 --> 00:14:15,900
we're running the experiment.

296
00:14:15,900 --> 00:14:19,830
So we set the value of
the incident energy

297
00:14:19,830 --> 00:14:21,640
of the photon beam.

298
00:14:21,640 --> 00:14:24,200
And then we send this
to a detector.

299
00:14:24,200 --> 00:14:27,210
This goes to a detector.

300
00:14:27,210 --> 00:14:30,930
And the detector measures the
velocity and ultimately gives

301
00:14:30,930 --> 00:14:33,980
us the energy of the
scattered electron.

302
00:14:33,980 --> 00:14:38,820
This is called the dislodged
electron, or photoelectron.

303
00:14:38,820 --> 00:14:41,530
It's the electron that was
kicked out by the photon.

304
00:14:41,530 --> 00:14:43,670
So it's known as the
photoelectron.

305
00:14:47,810 --> 00:14:49,200
Photoelectron.

306
00:14:49,200 --> 00:14:52,430
Pardon me, let's
try that again.

307
00:14:52,430 --> 00:15:05,170
Photoelectron energy is measured
at the detector.

308
00:15:05,170 --> 00:15:07,530
And then by difference, we
get the binding energy.

309
00:15:07,530 --> 00:15:09,230
We get the binding energy.

310
00:15:09,230 --> 00:15:11,340
So this is h nu.

311
00:15:11,340 --> 00:15:16,550
This one here is going to be a
1/2 m b squared, and then by

312
00:15:16,550 --> 00:15:19,160
difference we get the
binding energy.

313
00:15:19,160 --> 00:15:22,790
And what kind of photon
energies do we need?

314
00:15:22,790 --> 00:15:24,370
We need fairly high energies.

315
00:15:24,370 --> 00:15:30,440
And so typically, we might use
wavelengths down around 1

316
00:15:30,440 --> 00:15:35,100
angstrom, which then
makes it an X-ray.

317
00:15:35,100 --> 00:15:38,800
And so, over here in Building
13, we have such

318
00:15:38,800 --> 00:15:41,750
instrumentation, and the
material scientists

319
00:15:41,750 --> 00:15:43,640
would call this XPS.

320
00:15:43,640 --> 00:15:46,580
X-ray photoelectron
spectroscopy.

321
00:15:46,580 --> 00:15:49,280
And if you don't want to blast
all of the electrons out of

322
00:15:49,280 --> 00:15:52,980
the specimen and just get the
most weakly bound, then you

323
00:15:52,980 --> 00:15:57,080
want a lower incident photon
energy, which means a longer

324
00:15:57,080 --> 00:15:58,000
wavelength.

325
00:15:58,000 --> 00:16:02,980
You might come in at around
100 angstroms, which is

326
00:16:02,980 --> 00:16:04,030
ultraviolet.

327
00:16:04,030 --> 00:16:06,980
You know, people, the general
public, is so afraid, they

328
00:16:06,980 --> 00:16:09,090
don't know science.

329
00:16:09,090 --> 00:16:10,990
If they hear X-ray they
get all panicked.

330
00:16:10,990 --> 00:16:12,320
You know, it's radiation.

331
00:16:12,320 --> 00:16:13,460
Their children are going
to die and all

332
00:16:13,460 --> 00:16:14,150
this kind of stuff.

333
00:16:14,150 --> 00:16:17,150
So, what people will do is,
they'll say instead of soft

334
00:16:17,150 --> 00:16:19,730
X-ray they'll say hard
ultraviolet.

335
00:16:19,730 --> 00:16:21,730
And then the public thinks, oh,
as long I wear sunscreen

336
00:16:21,730 --> 00:16:24,330
I'm going to be OK.

337
00:16:24,330 --> 00:16:27,330
So if you use hard ultraviolet,
it's called UPS.

338
00:16:27,330 --> 00:16:28,860
That's nothing to do
with brown trucks.

339
00:16:28,860 --> 00:16:33,990
It's ultraviolet photoelectron
spectroscopy.

340
00:16:33,990 --> 00:16:37,200
And collectively, this
is known as PES--

341
00:16:37,200 --> 00:16:39,890
photoelectron spectroscopy.

342
00:16:39,890 --> 00:16:40,270
All right.

343
00:16:40,270 --> 00:16:42,970
So now, let's go back
to this schematic.

344
00:16:42,970 --> 00:16:46,710
Now you can see how bad this
schematic was that these

345
00:16:46,710 --> 00:16:49,420
photons are coming in as so.

346
00:16:49,420 --> 00:16:54,470
And striking the specimen
here, ejecting the

347
00:16:54,470 --> 00:16:57,460
photoelectrons which then are
focused and ultimately

348
00:16:57,460 --> 00:16:58,620
measured here at the detector.

349
00:16:58,620 --> 00:17:00,730
So there's the energy balance.

350
00:17:00,730 --> 00:17:03,780
The energy of the incident
photon is distributed across

351
00:17:03,780 --> 00:17:06,960
the kinetic energy of the
photoelectron plus the binding

352
00:17:06,960 --> 00:17:09,400
energy, and this is how we
measure all of these

353
00:17:09,400 --> 00:17:11,650
quantities.

354
00:17:11,650 --> 00:17:15,690
Since you're using very, very
high intensity radiation,

355
00:17:15,690 --> 00:17:17,030
there's nothing saying
that you're going to

356
00:17:17,030 --> 00:17:19,110
eject only one electron.

357
00:17:19,110 --> 00:17:22,630
You can eject a plurality
of electrons.

358
00:17:22,630 --> 00:17:25,260
And the different
electrons have

359
00:17:25,260 --> 00:17:26,980
different binding energies.

360
00:17:26,980 --> 00:17:30,470
So you're going to get a set
of binding energies, which

361
00:17:30,470 --> 00:17:32,250
means you'll get
a set of lines.

362
00:17:32,250 --> 00:17:34,410
You'll get a spectrum
of energies.

363
00:17:34,410 --> 00:17:38,250
So this is what the spectrum
looks like, and this is, by

364
00:17:38,250 --> 00:17:43,220
the way, just a general way of
looking at any spectrum so

365
00:17:43,220 --> 00:17:47,120
that you train you eye to
know what to look for.

366
00:17:47,120 --> 00:17:51,910
If I look at any spectrum, I'm
going to have a plot of some

367
00:17:51,910 --> 00:17:57,660
kind of intensity, some kind of
intensity unit versus some

368
00:17:57,660 --> 00:18:02,390
kind of energy unit, versus
some kind of energy unit.

369
00:18:02,390 --> 00:18:06,030
And sometimes, energy increases
from left to right.

370
00:18:06,030 --> 00:18:08,720
But sometimes they might put
energy increasing from right

371
00:18:08,720 --> 00:18:10,320
to left as you see in
these spectrum.

372
00:18:10,320 --> 00:18:12,290
But energy is along here.

373
00:18:12,290 --> 00:18:17,790
And intensity is related
to population.

374
00:18:17,790 --> 00:18:22,150
All other things being equal, if
I have a specimen that has

375
00:18:22,150 --> 00:18:25,740
two times the concentration of
active species, I'm going to

376
00:18:25,740 --> 00:18:29,180
get twice the strength
of the line.

377
00:18:29,180 --> 00:18:33,510
So the intensity measures
population.

378
00:18:33,510 --> 00:18:37,750
The energy is related to the
characteristic binding energy

379
00:18:37,750 --> 00:18:41,090
of a particular electrons, so
the energy is related to the

380
00:18:41,090 --> 00:18:43,620
identity of the species.

381
00:18:43,620 --> 00:18:45,840
This is how we can identify
certain species.

382
00:18:45,840 --> 00:18:48,990
I've already shown you if you
walk in and you see those four

383
00:18:48,990 --> 00:18:52,360
Balmer series lines on a
spectrum, you know that's

384
00:18:52,360 --> 00:18:54,130
characteristic of
atomic hydrogen.

385
00:18:54,130 --> 00:18:55,510
Nothing else.

386
00:18:55,510 --> 00:18:57,650
So those are energies.

387
00:18:57,650 --> 00:18:59,460
And then, where do we go?

388
00:18:59,460 --> 00:19:03,950
So we have a line here, but we
know that real data have a

389
00:19:03,950 --> 00:19:05,190
little bit of a spread
to them.

390
00:19:05,190 --> 00:19:08,660
So you don't see discrete lines;
you'll see a spread

391
00:19:08,660 --> 00:19:11,360
centered at about the value.

392
00:19:11,360 --> 00:19:12,310
So let's look at these.

393
00:19:12,310 --> 00:19:15,490
These are highly stylized, but
here you can see-- here's

394
00:19:15,490 --> 00:19:16,950
boron for example.

395
00:19:16,950 --> 00:19:20,680
So these are the 1s electrons of
boron, and they're held at

396
00:19:20,680 --> 00:19:25,190
a binding energy of about 19.3
megajoules per mole.

397
00:19:25,190 --> 00:19:28,640
And then this is 2s,
and here's 2p.

398
00:19:28,640 --> 00:19:31,590
Now there's a difference between
2s and 2p, but they're

399
00:19:31,590 --> 00:19:32,680
of the same order.

400
00:19:32,680 --> 00:19:35,170
They're roughly about a
megajoule per mole.

401
00:19:35,170 --> 00:19:36,620
There's subtle differences.

402
00:19:36,620 --> 00:19:39,500
And this goes back to
Sommerfeld, who said that even

403
00:19:39,500 --> 00:19:44,410
though there are s and p
orbitals, there's a circular

404
00:19:44,410 --> 00:19:46,290
orbit and there's an
elliptical orbit.

405
00:19:46,290 --> 00:19:48,400
But they're roughly in
the same shell and

406
00:19:48,400 --> 00:19:49,450
that's what you see.

407
00:19:49,450 --> 00:19:53,240
But there's a huge difference in
going from shell n equals 1

408
00:19:53,240 --> 00:19:54,210
to n equals 2.

409
00:19:54,210 --> 00:19:55,080
This is not to scale.

410
00:19:55,080 --> 00:19:57,460
You see they got a little
break there.

411
00:19:57,460 --> 00:20:01,870
And now look, what's the
electronic structure of boron?

412
00:20:01,870 --> 00:20:05,350
1s2 2s2 2s1.

413
00:20:05,350 --> 00:20:10,890
There are two 2s electrons,
but only one 2p electron.

414
00:20:10,890 --> 00:20:14,050
And this is trying to show that
the height here is twice

415
00:20:14,050 --> 00:20:17,640
the height of the 2s peak.

416
00:20:17,640 --> 00:20:21,800
So you see that we've got two
electrons here, one electron

417
00:20:21,800 --> 00:20:27,540
here, and it's easiest to pull
out the 2p electrons first.

418
00:20:27,540 --> 00:20:30,830
And you go on, there's
beryllium and so on.

419
00:20:30,830 --> 00:20:36,460
So we see that we can build
data in this fashion here.

420
00:20:36,460 --> 00:20:37,350
We keep going.

421
00:20:37,350 --> 00:20:41,090
Here's carbon, which
is 2, 2, 2.

422
00:20:41,090 --> 00:20:42,590
And then oxygen has 2s2--

423
00:20:45,220 --> 00:20:45,550
pardon me.

424
00:20:45,550 --> 00:20:48,800
1s2, 2s2, 2p4.

425
00:20:48,800 --> 00:20:51,830
So this is two electrons,
this is four electrons.

426
00:20:51,830 --> 00:20:57,160
Showing that the 2p height
is twice the 2s height.

427
00:20:57,160 --> 00:21:00,290
And then finally, neon
is 2s2, 2p6.

428
00:21:00,290 --> 00:21:05,190
So this is roughly three times
the height of the 2p.

429
00:21:05,190 --> 00:21:09,800
But all of the 2p numbers are
roughly of the same order of

430
00:21:09,800 --> 00:21:13,300
magnitude and decidedly
smaller than

431
00:21:13,300 --> 00:21:15,250
what's going on at 1s.

432
00:21:15,250 --> 00:21:16,710
And what else do you see?

433
00:21:16,710 --> 00:21:20,490
Well, the carbon has 6
protons, so the inner

434
00:21:20,490 --> 00:21:24,570
electrons of carbon are held
not as tightly as the inner

435
00:21:24,570 --> 00:21:29,430
electrons of neon, which
has 10 protons.

436
00:21:29,430 --> 00:21:31,510
All of this comes out.

437
00:21:31,510 --> 00:21:33,720
The data are all good.

438
00:21:33,720 --> 00:21:35,650
So now what do we want
to talk about?

439
00:21:35,650 --> 00:21:36,580
What's all this about?

440
00:21:36,580 --> 00:21:39,360
I said electronic structure
leads to properties.

441
00:21:39,360 --> 00:21:41,490
The first thing we want to
talk about is reactivity,

442
00:21:41,490 --> 00:21:42,960
chemical reactivity.

443
00:21:42,960 --> 00:21:46,980
Well it's pretty clear from
looking at these XPS data that

444
00:21:46,980 --> 00:21:49,870
those inner shell electrons
aren't going anywhere.

445
00:21:49,870 --> 00:21:52,130
They're too tightly held.

446
00:21:52,130 --> 00:21:55,740
So first thing is the only
electrons that we can even

447
00:21:55,740 --> 00:21:59,970
think about involving in
chemical reactivity must be

448
00:21:59,970 --> 00:22:03,200
the electrons in the
outermost shell.

449
00:22:03,200 --> 00:22:05,950
These are called valence
electrons.

450
00:22:05,950 --> 00:22:09,320
The outermost shell is called
the valence shell.

451
00:22:09,320 --> 00:22:13,760
So let's get that down because
that's probably important.

452
00:22:13,760 --> 00:22:29,910
So chemical reactivity
determined by only electrons

453
00:22:29,910 --> 00:22:31,170
in outermost shell.

454
00:22:37,200 --> 00:22:41,340
And we're going to term that
the valence shell.

455
00:22:41,340 --> 00:22:44,175
And those electrons are called
the valence electrons.

456
00:22:48,950 --> 00:22:54,900
And, these are the only ones
that we expect to see involved

457
00:22:54,900 --> 00:22:56,250
in chemical reactivity.

458
00:22:56,250 --> 00:22:57,780
And we want to have a measure.

459
00:22:57,780 --> 00:23:00,300
You can see that there are
subtle differences between the

460
00:23:00,300 --> 00:23:04,480
energies that hold the valence
electrons of carbon as opposed

461
00:23:04,480 --> 00:23:06,370
the valence electrons of neon.

462
00:23:06,370 --> 00:23:16,380
And so what we can do is have
a measure of the ability for

463
00:23:16,380 --> 00:23:23,250
valence electrons to react.

464
00:23:23,250 --> 00:23:27,330
That is to say, to participate
in chemical activity.

465
00:23:27,330 --> 00:23:30,840
And that measure is
called the average

466
00:23:30,840 --> 00:23:32,570
valence electron energy.

467
00:23:37,930 --> 00:23:40,065
And there's no special symbol
for that, so we

468
00:23:40,065 --> 00:23:41,940
just call it AVEE.

469
00:23:41,940 --> 00:23:46,190
And it's just the values
of the valence

470
00:23:46,190 --> 00:23:47,330
electron energies averaged.

471
00:23:47,330 --> 00:23:50,210
So if I want to take the average
valence electron

472
00:23:50,210 --> 00:23:52,310
energy for say, oxygen.

473
00:23:52,310 --> 00:23:54,030
So I can take it right
off of there.

474
00:23:54,030 --> 00:23:58,280
I can see I'm going to need
two times the ionization

475
00:23:58,280 --> 00:24:10,940
energy of 2s plus four times the
ionization energy of 2p.

476
00:24:10,940 --> 00:24:12,250
And those data are up there.

477
00:24:12,250 --> 00:24:13,880
All divided by 6.

478
00:24:13,880 --> 00:24:18,530
2 plus 4, the total number
of valence electrons.

479
00:24:18,530 --> 00:24:26,110
And if I go through the math up
there I get 1.91 megajoules

480
00:24:26,110 --> 00:24:32,660
per mole, which I prefer to
expresses as 19.8 electron

481
00:24:32,660 --> 00:24:36,060
volts per atom.

482
00:24:36,060 --> 00:24:39,530
And when we go through and
calculate the values of

483
00:24:39,530 --> 00:24:43,960
average valence electron
energies, we find trends.

484
00:24:43,960 --> 00:24:46,030
And here's what the
trends look like.

485
00:24:46,030 --> 00:24:50,320
So you have, first of all,
following along with the

486
00:24:50,320 --> 00:24:53,460
ionization energies, very
similar values.

487
00:24:53,460 --> 00:24:54,380
Pardon me.

488
00:24:54,380 --> 00:24:58,840
So here's the plot of average
valence electron energy in bar

489
00:24:58,840 --> 00:25:02,530
heights arranged as the elements
are found on the

490
00:25:02,530 --> 00:25:05,380
Periodic Table.

491
00:25:05,380 --> 00:25:09,460
So you have hydrogen here,
helium on this scale

492
00:25:09,460 --> 00:25:10,480
would be way up.

493
00:25:10,480 --> 00:25:12,810
And then here's lithium,
beryllium, boron.

494
00:25:12,810 --> 00:25:16,730
And so you see a monotonic
increase as you move

495
00:25:16,730 --> 00:25:18,470
from left to right.

496
00:25:18,470 --> 00:25:21,310
And you see a monotonic decrease
if you stay in the

497
00:25:21,310 --> 00:25:27,050
same column from low
mass to high mass.

498
00:25:27,050 --> 00:25:28,170
So what's going on there?

499
00:25:28,170 --> 00:25:30,250
Why do we have that change?

500
00:25:30,250 --> 00:25:33,030
We have similar electronics
structures.

501
00:25:33,030 --> 00:25:36,650
We have similar electronics
structures in a given column

502
00:25:36,650 --> 00:25:42,340
and lithium is s1, sodium
is s1, potassium is

503
00:25:42,340 --> 00:25:45,440
s1, rubidium is s1.

504
00:25:45,440 --> 00:25:48,010
And let's put some
values on here.

505
00:25:48,010 --> 00:25:51,240
Let's put some values.

506
00:25:51,240 --> 00:25:55,410
For the majority of elements in
the Periodic Table, we get

507
00:25:55,410 --> 00:26:01,350
values of average valence
electron energy less than 11

508
00:26:01,350 --> 00:26:03,290
electron volts.

509
00:26:03,290 --> 00:26:06,920
And when we have such values,
we have the following

510
00:26:06,920 --> 00:26:11,450
properties: the valence
electrons are weakly held.

511
00:26:11,450 --> 00:26:14,300
The valence electrons
are weakly held.

512
00:26:14,300 --> 00:26:15,000
How do we know this?

513
00:26:15,000 --> 00:26:17,150
Because the binding energy
isn't so strong.

514
00:26:17,150 --> 00:26:19,890
That means the electronic
are weakly held.

515
00:26:19,890 --> 00:26:24,080
So we say that the element
is a good electron donor.

516
00:26:27,252 --> 00:26:31,150
It's a good electron donor
and we term this a metal.

517
00:26:31,150 --> 00:26:34,870
The property that makes an
element a metal is that it has

518
00:26:34,870 --> 00:26:39,110
a low value of average valence
electron energy and therefore,

519
00:26:39,110 --> 00:26:41,280
it is a good electron donor.

520
00:26:41,280 --> 00:26:44,600
And it turns out about
75% of the Periodic

521
00:26:44,600 --> 00:26:48,210
Table is made of metals.

522
00:26:48,210 --> 00:26:53,410
At the other extreme we have
high values of average valence

523
00:26:53,410 --> 00:26:54,650
electron energy.

524
00:26:54,650 --> 00:26:56,200
High values.

525
00:26:56,200 --> 00:26:59,940
So when the average valence
electron energy is high, we

526
00:26:59,940 --> 00:27:03,700
have the complementary
set of properties.

527
00:27:03,700 --> 00:27:07,750
So that means the valence
electrons are tightly held.

528
00:27:07,750 --> 00:27:09,675
Valence electrons
tightly held.

529
00:27:12,430 --> 00:27:13,630
Tightly held.

530
00:27:13,630 --> 00:27:17,790
And so the element is a
poor electron donor.

531
00:27:22,790 --> 00:27:27,610
But complementary fashion, it
is a good electron acceptor

532
00:27:27,610 --> 00:27:31,190
because when it gets near
electrons it tends to grab

533
00:27:31,190 --> 00:27:32,750
them and hold on to them.

534
00:27:32,750 --> 00:27:36,440
So this is a good electron
acceptor.

535
00:27:39,190 --> 00:27:44,100
And we term such a element
a nonmetal.

536
00:27:44,100 --> 00:27:50,190
And so roughly, about 25%
of the Periodic Table is

537
00:27:50,190 --> 00:27:52,380
non-metallic.

538
00:27:52,380 --> 00:27:54,770
And I think I've got
that shown here.

539
00:27:54,770 --> 00:27:57,330
And you see just a
few elements off

540
00:27:57,330 --> 00:27:59,980
to the upper right.

541
00:27:59,980 --> 00:28:02,540
And then in the middle,
we've got this--

542
00:28:02,540 --> 00:28:07,800
about a half a dozen elements
and they have values of

543
00:28:07,800 --> 00:28:15,630
average valence electron energy
intermediate between 11

544
00:28:15,630 --> 00:28:17,860
and 13 electron volts.

545
00:28:17,860 --> 00:28:23,500
And so, they can behave either
as electron donors or electron

546
00:28:23,500 --> 00:28:27,040
acceptors depending on who
else is in the room.

547
00:28:27,040 --> 00:28:31,870
So if you put an element like
silicon with a very strong

548
00:28:31,870 --> 00:28:35,390
metal, silicon will act as
an electron acceptor.

549
00:28:35,390 --> 00:28:38,350
If you put silicon in the
presence of something that's a

550
00:28:38,350 --> 00:28:42,190
very strong nonmetal, silicon
can act as an electron donor.

551
00:28:42,190 --> 00:28:44,220
It has that dual property.

552
00:28:44,220 --> 00:28:51,290
And these elements are called
semimetals or metalloids.

553
00:28:57,610 --> 00:29:01,680
And I think I've got some
images to portray that.

554
00:29:01,680 --> 00:29:03,010
There's the semimetals.

555
00:29:03,010 --> 00:29:06,710
And if you look carefully at
the Periodic Table that you

556
00:29:06,710 --> 00:29:10,110
have, you'll see that there's
this red staircase here.

557
00:29:10,110 --> 00:29:15,400
So there's silicon, arsenic,
tellurium, antimony and so on

558
00:29:15,400 --> 00:29:19,050
that act as semimetals.

559
00:29:19,050 --> 00:29:24,040
They can behave in
both fashions.

560
00:29:24,040 --> 00:29:28,500
All right, so now with this
classification scheme, let's

561
00:29:28,500 --> 00:29:31,820
start thinking about chemical
reactivity.

562
00:29:31,820 --> 00:29:35,480
And seeing if we can, on the
basis of where the elements

563
00:29:35,480 --> 00:29:37,840
are found on the Periodic
Table, start

564
00:29:37,840 --> 00:29:39,790
to make some judgments.

565
00:29:39,790 --> 00:29:43,850
Well I know one thing right
off the bat that along the

566
00:29:43,850 --> 00:29:47,610
right-hand column, the noble
gases are chemically inert.

567
00:29:47,610 --> 00:29:48,870
That much we know.

568
00:29:48,870 --> 00:29:54,890
Noble gases are chemically
inert.

569
00:29:54,890 --> 00:30:00,390
I know there's a Nobel Prize out
there for getting xenon to

570
00:30:00,390 --> 00:30:03,390
combine with fluorine, and the
chemistry textbooks love to

571
00:30:03,390 --> 00:30:05,420
point out these exceptions.

572
00:30:05,420 --> 00:30:11,780
But by and large, the group 8
elements are chemically inert.

573
00:30:11,780 --> 00:30:15,610
So what do we know about their
electronic structure?

574
00:30:15,610 --> 00:30:18,840
They all have the
same electronic

575
00:30:18,840 --> 00:30:21,695
structure ns 2, np 6.

576
00:30:24,430 --> 00:30:26,090
With the exception of helium.

577
00:30:26,090 --> 00:30:29,880
Helium, because it's 1s 2.

578
00:30:29,880 --> 00:30:30,880
There's no p.

579
00:30:30,880 --> 00:30:37,740
So beside helium, starting with
neon, we've got 2s2, 2p6.

580
00:30:37,740 --> 00:30:42,685
Argon is 3s2, 3p6, and so on.

581
00:30:42,685 --> 00:30:46,200
n2 p6, it has 8 p electrons.

582
00:30:48,930 --> 00:30:50,780
It has a full valence shell.

583
00:30:54,450 --> 00:31:03,080
And because the last electrons
are p electrons, this is

584
00:31:03,080 --> 00:31:04,550
termed octet stability.

585
00:31:07,080 --> 00:31:11,030
There's something about having
a full shell that renders the

586
00:31:11,030 --> 00:31:13,620
system satisfied
energetically.

587
00:31:13,620 --> 00:31:15,265
And it doesn't want
to react anymore.

588
00:31:17,970 --> 00:31:21,250
So we say octet stability, and
if you want to be a wise guy,

589
00:31:21,250 --> 00:31:24,110
parenthetically you whisper
helium is due at stability.

590
00:31:24,110 --> 00:31:26,890
But you know, whatever.

591
00:31:26,890 --> 00:31:31,220
All right, so let's make this
octet stability a hypothesis

592
00:31:31,220 --> 00:31:33,560
and see how far we
can go with it.

593
00:31:33,560 --> 00:31:37,630
So the next one I want
to look at is sodium.

594
00:31:37,630 --> 00:31:42,800
Now sodium, it's got the
electronic structure 1s 2,

595
00:31:42,800 --> 00:31:49,780
2s2, 2p6, 3s1.

596
00:31:49,780 --> 00:31:54,790
Now, I really don't need to talk
about anything up to n

597
00:31:54,790 --> 00:31:57,160
equals 3 because this is
not valence electrons.

598
00:31:57,160 --> 00:32:00,870
I just put it up there for
completeness, but this is all

599
00:32:00,870 --> 00:32:02,960
we have to worry about.

600
00:32:02,960 --> 00:32:05,490
Now we know that sodium
is a metal.

601
00:32:05,490 --> 00:32:06,120
It's a metal.

602
00:32:06,120 --> 00:32:10,180
It's got an average valence
electron energy of about 5.2

603
00:32:10,180 --> 00:32:12,900
electron volts, which happens
to be the ionization energy.

604
00:32:12,900 --> 00:32:15,610
Because that's trivial; it's
only got one electron, so the

605
00:32:15,610 --> 00:32:17,460
average valence electron
energy is equal to the

606
00:32:17,460 --> 00:32:19,260
ionization energy.

607
00:32:19,260 --> 00:32:20,065
So it's a metal.

608
00:32:20,065 --> 00:32:22,330
So it's a good electron donor.

609
00:32:22,330 --> 00:32:24,100
But let's think about that.

610
00:32:24,100 --> 00:32:27,680
If we could get rid of that
electron in sodium, we could

611
00:32:27,680 --> 00:32:30,240
then turn it into something that
has the same electronic

612
00:32:30,240 --> 00:32:32,120
structure as neon.

613
00:32:32,120 --> 00:32:33,190
And why do we want to do that?

614
00:32:33,190 --> 00:32:36,340
Well because there seems to
be an energy well there.

615
00:32:36,340 --> 00:32:38,640
If we can make something
neon-like, then

616
00:32:38,640 --> 00:32:40,010
it's going to be happy.

617
00:32:40,010 --> 00:32:41,740
So let's do it.

618
00:32:41,740 --> 00:32:47,520
Let's write a reaction sodium to
lose an electron and become

619
00:32:47,520 --> 00:32:49,330
sodium plus.

620
00:32:49,330 --> 00:32:52,950
And sodium plus now
is this minus 3s1.

621
00:32:52,950 --> 00:32:57,210
So it's isoelectronic
with neon, and neon

622
00:32:57,210 --> 00:32:59,390
has the octet stability.

623
00:32:59,390 --> 00:33:01,590
But charge neutrality
forbids me to

624
00:33:01,590 --> 00:33:04,250
say, OK, lose an electron.

625
00:33:04,250 --> 00:33:05,800
It won't.

626
00:33:05,800 --> 00:33:09,200
The only way for sodium
to lose an electron

627
00:33:09,200 --> 00:33:12,950
is to find an acceptor.

628
00:33:12,950 --> 00:33:16,090
So if you put a good electron
donor in the presence of a

629
00:33:16,090 --> 00:33:20,080
good electron acceptor, then the
donor can give an electron

630
00:33:20,080 --> 00:33:24,390
to the acceptor and both profit
from the transaction.

631
00:33:24,390 --> 00:33:27,330
So, where are we going to find
a good electron acceptor?

632
00:33:27,330 --> 00:33:29,890
Well I just told you, the
nonmetals are the really good

633
00:33:29,890 --> 00:33:31,180
electron acceptors.

634
00:33:31,180 --> 00:33:35,480
So let's go zooming over to the
right-hand edge of that

635
00:33:35,480 --> 00:33:39,030
row and look at chlorine.

636
00:33:39,030 --> 00:33:41,440
We need an electron acceptor.

637
00:33:44,430 --> 00:33:49,920
So choose just for argument's
sake, chlorine.

638
00:33:49,920 --> 00:33:51,590
It's got an average
valence electron

639
00:33:51,590 --> 00:33:53,920
energy way, way up there.

640
00:33:53,920 --> 00:33:54,730
So let's go.

641
00:33:54,730 --> 00:33:59,200
Chlorine, it has the electronic
structure of neon

642
00:33:59,200 --> 00:34:03,980
plus 3s2, 3p5.

643
00:34:03,980 --> 00:34:07,210
So chlorine, if it could acquire
an electron, would

644
00:34:07,210 --> 00:34:10,720
then become isoelectronic
with argon.

645
00:34:10,720 --> 00:34:16,210
So imagine chlorine plus
electron becomes Cl minus and

646
00:34:16,210 --> 00:34:19,350
Cl minus is isoelectronic
with argon.

647
00:34:19,350 --> 00:34:20,950
So now I've got two elements.

648
00:34:20,950 --> 00:34:25,630
One that's one electron richer
than inert gas structure, and

649
00:34:25,630 --> 00:34:28,880
one that's one electron poorer
than inert gas structure.

650
00:34:28,880 --> 00:34:32,920
If I put them together, put them
both together in the same

651
00:34:32,920 --> 00:34:42,370
reactor, they will react and
electron transfer occurs via

652
00:34:42,370 --> 00:34:43,646
electron transfer.

653
00:34:48,930 --> 00:34:50,345
Chemical reaction occurs.

654
00:34:53,690 --> 00:34:57,490
Chemical reaction occurs through
electron transfer.

655
00:34:57,490 --> 00:35:01,900
And what's the purpose
of electron transfer?

656
00:35:01,900 --> 00:35:05,680
It's to achieve full
valence shell.

657
00:35:05,680 --> 00:35:12,360
Achieve full valence shell
occupancy, if you like.

658
00:35:12,360 --> 00:35:15,710
Achieve full valence
shell occupancy.

659
00:35:15,710 --> 00:35:19,110
So, now we've got it.

660
00:35:19,110 --> 00:35:20,640
But there's more.

661
00:35:20,640 --> 00:35:21,460
There's more.

662
00:35:21,460 --> 00:35:24,980
What happens after the electron
transfer step?

663
00:35:24,980 --> 00:35:28,030
We don't have neutral
species anymore.

664
00:35:28,030 --> 00:35:31,160
The sodiums have given up their
electrons to become

665
00:35:31,160 --> 00:35:33,340
sodium ions.

666
00:35:33,340 --> 00:35:36,440
And the chlorines have acquired
electrons to become

667
00:35:36,440 --> 00:35:38,100
chloride ions.

668
00:35:38,100 --> 00:35:39,720
And these are free.

669
00:35:39,720 --> 00:35:40,700
Let's make this simple.

670
00:35:40,700 --> 00:35:42,770
Let's make it a gas
phase reaction.

671
00:35:42,770 --> 00:35:47,140
So sodium vapor and chlorine
gas have become sodium ion

672
00:35:47,140 --> 00:35:49,110
vapor and chloride ion vapor.

673
00:35:49,110 --> 00:35:53,330
What do you know happens when
you have charges of opposite

674
00:35:53,330 --> 00:35:55,750
value, opposite polarity?

675
00:35:55,750 --> 00:35:57,980
There's a coulombic force
of attraction.

676
00:35:57,980 --> 00:36:02,540
So the sodium will attract
the chlorine.

677
00:36:02,540 --> 00:36:04,680
So let's put the two together.

678
00:36:04,680 --> 00:36:07,850
So sodium attracts chlorine,
but it's not over.

679
00:36:07,850 --> 00:36:10,320
There's more sodiums, there's
more chlorines.

680
00:36:10,320 --> 00:36:18,060
So chlorine attracts sodium,
which attracts sodium.

681
00:36:18,060 --> 00:36:20,810
Which could attract
more chlorines.

682
00:36:20,810 --> 00:36:23,470
Which could attract
more sodiums.

683
00:36:23,470 --> 00:36:25,630
And what do you see
happening here?

684
00:36:25,630 --> 00:36:30,660
Well we're now starting with a
gas phase and we're forming

685
00:36:30,660 --> 00:36:33,980
some giant atomic aggregate.

686
00:36:33,980 --> 00:36:38,895
This is a giant ion aggregate.

687
00:36:43,930 --> 00:36:46,300
What's the state of matter going
to be if this thing's

688
00:36:46,300 --> 00:36:48,170
honking big?

689
00:36:48,170 --> 00:36:50,620
It's going to be a solid.

690
00:36:50,620 --> 00:36:53,920
We're going to form a solid
starting with those two gases:

691
00:36:53,920 --> 00:36:56,320
sodium vapor and
chlorine vapor.

692
00:36:56,320 --> 00:36:57,600
And what about the solid?

693
00:36:57,600 --> 00:37:00,280
What do we see about the atomic
arrangement here?

694
00:37:00,280 --> 00:37:03,890
All the sodiums are the same
size and all the chlorides are

695
00:37:03,890 --> 00:37:05,120
the same size.

696
00:37:05,120 --> 00:37:08,300
They have the same coulombic
forces of attraction in

697
00:37:08,300 --> 00:37:09,210
between them.

698
00:37:09,210 --> 00:37:11,910
Can you see that they're going
to form not just a solid, but

699
00:37:11,910 --> 00:37:15,480
they're going to form a solid
consisting of atoms in a

700
00:37:15,480 --> 00:37:18,200
regular array?

701
00:37:18,200 --> 00:37:20,085
This is going to be
an ordered solid.

702
00:37:23,610 --> 00:37:25,830
And there's a plain Anglo-Saxon
word for an

703
00:37:25,830 --> 00:37:28,010
ordered solid, it's
called a crystal.

704
00:37:31,500 --> 00:37:33,970
Now look at how far
we've come.

705
00:37:33,970 --> 00:37:36,030
Look at how far we've come.

706
00:37:36,030 --> 00:37:39,410
10 minutes ago we hadn't done
anything about chemical

707
00:37:39,410 --> 00:37:40,650
reactivity.

708
00:37:40,650 --> 00:37:42,210
We made one observation.

709
00:37:42,210 --> 00:37:43,560
We had the tools of course.

710
00:37:43,560 --> 00:37:44,590
We were ready.

711
00:37:44,590 --> 00:37:46,140
We're ready for discovery.

712
00:37:46,140 --> 00:37:48,170
We had the tools.

713
00:37:48,170 --> 00:37:50,830
We had the average valence
electron energy.

714
00:37:50,830 --> 00:37:54,140
We knew that helium, neon,
argon, krypton, xenon, radon,

715
00:37:54,140 --> 00:37:58,220
et cetera, all have
this inertness.

716
00:37:58,220 --> 00:38:02,910
We postulated that the octet
stability was some--

717
00:38:02,910 --> 00:38:04,420
as a sweet spot.

718
00:38:04,420 --> 00:38:06,060
And then we went with it.

719
00:38:06,060 --> 00:38:10,210
And operating with that one
postulate, we get to electron

720
00:38:10,210 --> 00:38:13,850
transfer and we're concluding
that if I take a really,

721
00:38:13,850 --> 00:38:15,760
really good metal--

722
00:38:15,760 --> 00:38:17,820
see, I can generalize
this now.

723
00:38:17,820 --> 00:38:21,880
There's nothing peculiar about
sodium that makes it react

724
00:38:21,880 --> 00:38:25,280
this way and no other element
will react this way.

725
00:38:25,280 --> 00:38:29,360
I could replace sodium
with any other metal.

726
00:38:29,360 --> 00:38:34,160
I could say any metal, any
metal will react with any

727
00:38:34,160 --> 00:38:38,500
nonmetal in order to achieve
octet stability.

728
00:38:38,500 --> 00:38:43,290
So now you can take big pieces
of the Periodic Table and

729
00:38:43,290 --> 00:38:48,720
conclude that entire sets of
elements will act in this way

730
00:38:48,720 --> 00:38:54,400
to form ionic compounds,
crystals, atoms of regular

731
00:38:54,400 --> 00:38:56,450
periodicity.

732
00:38:56,450 --> 00:38:58,200
That's a nice pun
there, right?

733
00:38:58,200 --> 00:39:01,100
We started with the Periodic
Table and now we have periodic

734
00:39:01,100 --> 00:39:04,690
spacing of atoms. I think
that's beautiful.

735
00:39:04,690 --> 00:39:07,830
Obviously this view
is not shared by

736
00:39:07,830 --> 00:39:10,250
the rest of the audience.

737
00:39:10,250 --> 00:39:14,070
This crystal consists of ions.

738
00:39:14,070 --> 00:39:16,520
It's held together by
what chemists like

739
00:39:16,520 --> 00:39:19,490
to refer to as bonds.

740
00:39:19,490 --> 00:39:22,720
This is called an ionic bond.

741
00:39:22,720 --> 00:39:27,340
And what you're witnessing here
is ironic bonding via

742
00:39:27,340 --> 00:39:28,805
electron transfer.

743
00:39:31,330 --> 00:39:36,050
So we've now categorized the
first form of primary bonding.

744
00:39:39,870 --> 00:39:42,610
Sodium chloride, magnesium
oxide--

745
00:39:42,610 --> 00:39:43,790
ionic bonding.

746
00:39:43,790 --> 00:39:47,405
And ionic bonding must give us
solids at room temperature.

747
00:39:53,540 --> 00:39:56,440
If you've got ionic bonds and
you don't have a solid,

748
00:39:56,440 --> 00:40:00,710
clearly you are at elevated
temperature where at elevated

749
00:40:00,710 --> 00:40:03,980
temperature, the thermal energy
is disruptive enough to

750
00:40:03,980 --> 00:40:04,730
break those bonds.

751
00:40:04,730 --> 00:40:06,240
Those are very, very
strong bonds.

752
00:40:06,240 --> 00:40:10,660
And what we're going to do next
day is look at the nature

753
00:40:10,660 --> 00:40:14,060
of those bonds.

754
00:40:14,060 --> 00:40:18,580
So I think I'm going
to stop the formal

755
00:40:18,580 --> 00:40:20,270
lesson at this point.

756
00:40:20,270 --> 00:40:26,480
And by the way, perhaps I failed
to mention on the first

757
00:40:26,480 --> 00:40:27,240
day of the class.

758
00:40:27,240 --> 00:40:31,190
What I'm doing is I'm lecturing
for, out of the 50

759
00:40:31,190 --> 00:40:36,650
minutes, about 45 on
hard core topics.

760
00:40:36,650 --> 00:40:40,990
And then, about the last 5,
7 minutes, I want to go to

761
00:40:40,990 --> 00:40:42,990
something related to these--
chemistry in the

762
00:40:42,990 --> 00:40:44,130
world around us.

763
00:40:44,130 --> 00:40:47,080
That's why you see this
little break.

764
00:40:47,080 --> 00:40:48,800
But it doesn't mean that
class is dismissed.

765
00:40:48,800 --> 00:40:50,930
There's a difference between
changing topics

766
00:40:50,930 --> 00:40:51,910
and dismissing class.

767
00:40:51,910 --> 00:40:55,130
I don't know how people
confuse the two.

768
00:40:55,130 --> 00:40:59,490
But somehow I've inadvertently
communicated to you evidently.

769
00:40:59,490 --> 00:41:01,360
So we're not dismissing yet.

770
00:41:01,360 --> 00:41:05,650
We're going to try to apply
this knowledge somehow.

771
00:41:05,650 --> 00:41:10,450
So what I wanted to do was to
show you what happens with

772
00:41:10,450 --> 00:41:13,990
these ionic solids
in commerce.

773
00:41:13,990 --> 00:41:18,230
And in particular, I wanted to
talk about metallurgy and the

774
00:41:18,230 --> 00:41:20,100
best form of metallurgy,
which is of course,

775
00:41:20,100 --> 00:41:21,540
electrometallurgy.

776
00:41:21,540 --> 00:41:23,730
Which is the kind of research
that I'm involved in.

777
00:41:23,730 --> 00:41:26,030
And electrometallurgy
is quite pervasive.

778
00:41:26,030 --> 00:41:31,930
The aluminum beverage can is
made by electrometallurgy, as

779
00:41:31,930 --> 00:41:33,645
is magnesium.

780
00:41:33,645 --> 00:41:35,560
So I'm going to talk to you a
little bit about magnesium.

781
00:41:35,560 --> 00:41:37,340
This is a bar of magnesium.

782
00:41:37,340 --> 00:41:40,090
Obviously, it's magnesium
because if it were steel, it

783
00:41:40,090 --> 00:41:42,395
would be so massive that I
wouldn't be able to fling it

784
00:41:42,395 --> 00:41:43,500
around like this.

785
00:41:43,500 --> 00:41:49,160
The density of iron is 7.87,
the density of magnesium is

786
00:41:49,160 --> 00:41:51,110
about 1.76.

787
00:41:51,110 --> 00:41:53,310
It's less than 2.

788
00:41:53,310 --> 00:41:55,040
It's 2/3 of the density
of aluminum.

789
00:41:55,040 --> 00:41:58,290
It's lighter than aluminum.

790
00:41:58,290 --> 00:42:00,140
Now perhaps some of
you've been told

791
00:42:00,140 --> 00:42:01,230
that magnesium burns.

792
00:42:01,230 --> 00:42:02,810
Well, let's see what happens.

793
00:42:02,810 --> 00:42:05,020
There's a billet of magnesium
and this is

794
00:42:05,020 --> 00:42:08,620
the key made of steel.

795
00:42:08,620 --> 00:42:09,812
We're OK.

796
00:42:09,812 --> 00:42:10,460
We're OK.

797
00:42:10,460 --> 00:42:12,590
It's not going to burn.

798
00:42:12,590 --> 00:42:13,700
That's a myth.

799
00:42:13,700 --> 00:42:15,780
Magnesium in powder
form, magnesium in

800
00:42:15,780 --> 00:42:17,510
ribbon form will burn.

801
00:42:17,510 --> 00:42:20,760
The surface to volume ratio is
so high that the oxidation

802
00:42:20,760 --> 00:42:21,840
generates a lot of heat.

803
00:42:21,840 --> 00:42:24,830
But here the surface to
volume ratio is so

804
00:42:24,830 --> 00:42:26,620
low that it's fine.

805
00:42:26,620 --> 00:42:29,870
In fact, I went to school at
the University of Toronto.

806
00:42:29,870 --> 00:42:32,230
And at the time the department
head was a man by the name of

807
00:42:32,230 --> 00:42:35,520
Pigeon who invented a process
during World War II to make

808
00:42:35,520 --> 00:42:36,890
magnesium cheaply.

809
00:42:36,890 --> 00:42:40,440
And when he came into the
classroom to lecture on the

810
00:42:40,440 --> 00:42:43,990
day that he would talk about
magnesium, in those days they

811
00:42:43,990 --> 00:42:47,740
had lab benches at the front
of the class with sinks and

812
00:42:47,740 --> 00:42:48,750
gas outlets.

813
00:42:48,750 --> 00:42:55,000
And he would put the magnesium
billet on a stand and light a

814
00:42:55,000 --> 00:42:56,490
Bunsen burner under it.

815
00:42:56,490 --> 00:42:59,960
And of course, people in the
class sort of leaned back, and

816
00:42:59,960 --> 00:43:02,860
he would lecture for 45 five
minutes with the Bunsen flame

817
00:43:02,860 --> 00:43:05,220
burning the whole time.

818
00:43:05,220 --> 00:43:08,660
And then towards the end he
would extinguish the flame,

819
00:43:08,660 --> 00:43:10,560
let the billet cool.

820
00:43:10,560 --> 00:43:13,430
And then pick it up and go back
to his office with it to

821
00:43:13,430 --> 00:43:14,890
make the point.

822
00:43:14,890 --> 00:43:18,010
All right, so how do we
make these metals?

823
00:43:18,010 --> 00:43:22,250
Well, what we do is we reverse
the electron transfer step.

824
00:43:22,250 --> 00:43:26,680
And we start with the cations
and anions, and we force the

825
00:43:26,680 --> 00:43:31,520
electrons back onto the
cations anions.

826
00:43:31,520 --> 00:43:34,170
Thereby, creating the
neutrals again.

827
00:43:34,170 --> 00:43:35,560
This is called electrolysis.

828
00:43:35,560 --> 00:43:38,530
So cation plus electron
gives us neutral.

829
00:43:38,530 --> 00:43:40,980
Anion minus electron
gives us neutral.

830
00:43:40,980 --> 00:43:45,000
So in the case of magnesium,
we start with magnesium

831
00:43:45,000 --> 00:43:49,790
chloride, which dissolves and
forms magnesium cations and

832
00:43:49,790 --> 00:43:51,130
chloride anions.

833
00:43:51,130 --> 00:43:53,190
So, at elevated temperature--

834
00:43:53,190 --> 00:43:56,900
in this case, at about 700
degrees Celsius, the ionic

835
00:43:56,900 --> 00:44:00,600
solid becomes an ionic liquid.

836
00:44:00,600 --> 00:44:05,880
And it's clear, colorless, and
has the fluidity of water.

837
00:44:05,880 --> 00:44:08,950
And this is a very simple
schematic, so we've made the

838
00:44:08,950 --> 00:44:12,930
cathode of steel negative and
on the negative electrode,

839
00:44:12,930 --> 00:44:15,710
magnesium ions are reduced to
magnesium, which is a liquid

840
00:44:15,710 --> 00:44:16,500
at this temperature.

841
00:44:16,500 --> 00:44:17,920
And these are little bubbles
of magnesium

842
00:44:17,920 --> 00:44:19,125
and they pool here.

843
00:44:19,125 --> 00:44:20,990
And we collect them and
syphon them off.

844
00:44:20,990 --> 00:44:22,120
And on the other electrode,
we're

845
00:44:22,120 --> 00:44:23,770
making bubbles of chlorine.

846
00:44:23,770 --> 00:44:27,150
And they rise and
we collect them.

847
00:44:27,150 --> 00:44:29,322
So we reverse nature.

848
00:44:29,322 --> 00:44:32,180
If you put chlorine in the
presence of magnesium it wants

849
00:44:32,180 --> 00:44:33,320
to form magnesium chloride.

850
00:44:33,320 --> 00:44:36,080
Here we do the reverse.

851
00:44:36,080 --> 00:44:39,950
So electrolysis undoes the
spontaneous electron transfer

852
00:44:39,950 --> 00:44:41,080
of ion formations.

853
00:44:41,080 --> 00:44:44,060
So I decided I better get
something more authoritative

854
00:44:44,060 --> 00:44:45,070
than my little cartoon.

855
00:44:45,070 --> 00:44:49,540
So first I looked on the library
website and I found

856
00:44:49,540 --> 00:44:52,960
that there was an article in
this volume of Advances in

857
00:44:52,960 --> 00:44:54,410
Molten Salt Chemistry.

858
00:44:54,410 --> 00:44:58,930
And there's this article here,
an authoritative article about

859
00:44:58,930 --> 00:45:02,160
chemistry and electrochemistry
of magnesium production.

860
00:45:02,160 --> 00:45:08,350
And so I decided to turn to the
article and let me read

861
00:45:08,350 --> 00:45:12,800
the first paragraph of this
authoritative article.

862
00:45:12,800 --> 00:45:14,120
A cubic kilometer--

863
00:45:14,120 --> 00:45:15,190
what is a cubic kilometer?

864
00:45:15,190 --> 00:45:18,450
It's a cube, 1 kilometer
on edge.

865
00:45:18,450 --> 00:45:22,270
A cubic kilometer of sea water
contains approximately 1

866
00:45:22,270 --> 00:45:24,650
million tons of magnesium.

867
00:45:24,650 --> 00:45:29,220
1 million tons of magnesium as
the salt magnesium chloride

868
00:45:29,220 --> 00:45:31,740
decahydrate dissolved
in sea water.

869
00:45:31,740 --> 00:45:34,840
More than has ever been produced
in one year by all

870
00:45:34,840 --> 00:45:36,985
the magnesium plants
in the world.

871
00:45:36,985 --> 00:45:39,600
The world production of
magnesium right now is about

872
00:45:39,600 --> 00:45:44,030
600 million tons.

873
00:45:44,030 --> 00:45:48,690
No, furthermore, sea water
contains only 3.7% of the

874
00:45:48,690 --> 00:45:54,230
total magnesium present in the
earth's crust. Clearly

875
00:45:54,230 --> 00:45:56,890
magnesium resources are
ubiquitous and virtually

876
00:45:56,890 --> 00:45:59,170
inexhaustible.

877
00:45:59,170 --> 00:46:02,280
So when people tell you we've
got to recycle because we're

878
00:46:02,280 --> 00:46:05,960
running out of resources,
I read this--

879
00:46:05,960 --> 00:46:07,410
I don't know.

880
00:46:07,410 --> 00:46:09,810
You might want to recycle for
other reasons, but you can't

881
00:46:09,810 --> 00:46:11,320
make the case it's because
of scarcity.

882
00:46:13,940 --> 00:46:16,440
This is magnesium that
has been made by this

883
00:46:16,440 --> 00:46:17,320
electrolytic root.

884
00:46:17,320 --> 00:46:19,140
And what's the value of this?

885
00:46:19,140 --> 00:46:23,090
It can substitute for steel
in automobiles.

886
00:46:23,090 --> 00:46:26,080
It's density is less than 2.

887
00:46:26,080 --> 00:46:28,430
Steel is about 8.

888
00:46:28,430 --> 00:46:32,420
Factor of 4 you lightweight the
vehicle, which means per

889
00:46:32,420 --> 00:46:35,730
unit distance traveled,
less fuel consumed.

890
00:46:35,730 --> 00:46:38,140
Fewer emissions.

891
00:46:38,140 --> 00:46:40,750
And less fuel, which means less

892
00:46:40,750 --> 00:46:43,770
dependence on imported petroleum.

893
00:46:43,770 --> 00:46:45,420
So why aren't we doing this?

894
00:46:45,420 --> 00:46:48,690
Why aren't we lightweighting our
vehicles with magnesium.

895
00:46:48,690 --> 00:46:52,610
Because the bonds are so strong
the energy required to

896
00:46:52,610 --> 00:46:58,650
make this metal is so high
that it's very costly.

897
00:46:58,650 --> 00:47:02,570
If I give you a light bulb that
costs three times what

898
00:47:02,570 --> 00:47:06,300
the other light bulb does and
burns only twice as long, you

899
00:47:06,300 --> 00:47:07,720
won't buy it.

900
00:47:07,720 --> 00:47:10,630
If it costs three times as much
and burns 10 times as

901
00:47:10,630 --> 00:47:13,680
long, then you're willing
to pay the premium.

902
00:47:13,680 --> 00:47:17,640
So what we do in my research
group, among other things, is

903
00:47:17,640 --> 00:47:21,640
study the electrochemical
processes by which we make

904
00:47:21,640 --> 00:47:25,580
these metals in order to make
the process more efficient.

905
00:47:25,580 --> 00:47:29,200
Thereby reducing the cost,
making these lighter materials

906
00:47:29,200 --> 00:47:30,830
more competitive.

907
00:47:30,830 --> 00:47:36,710
Thereby, making the world a
better place by electron

908
00:47:36,710 --> 00:47:39,905
transfer in service
of humanity.

909
00:47:39,905 --> 00:47:41,960
And I invite you
to do the same.

910
00:47:41,960 --> 00:47:44,350
But in order to do so,
you have to learn

911
00:47:44,350 --> 00:47:45,990
the lessons of 3091.

912
00:47:45,990 --> 00:47:48,800
Because after all, this is the
most important subject you

913
00:47:48,800 --> 00:47:50,130
will take at MIT.

914
00:47:50,130 --> 00:47:52,090
All right, we'll see
you on Friday.