1
00:00:00,000 --> 00:00:00,024
The following content is
provided under a Creative

2
00:00:00,024 --> 00:00:00,033
Commons license.

3
00:00:00,033 --> 00:00:00,057
Your support will help MIT
OpenCourseWare continue to

4
00:00:00,057 --> 00:00:00,081
offer high quality educational
resources for free.

5
00:00:00,081 --> 00:00:00,108
To make a donation or view
additional materials from

6
00:00:00,108 --> 00:00:00,132
hundreds of MIT courses, visit
MIT OpenCourseWare at

7
00:00:00,132 --> 00:00:00,150
ocw.mit.edu.

8
00:00:00,150 --> 00:00:21,270
PROFESSOR: --

9
00:00:21,270 --> 00:00:23,750
10 seconds to answer the
first clicker question.

10
00:00:23,750 --> 00:00:27,590
As you can see we're having
another competition today, so

11
00:00:27,590 --> 00:00:30,680
see if you can beat out Justin's
recitation and get

12
00:00:30,680 --> 00:00:33,130
the most correct today.

13
00:00:33,130 --> 00:00:38,310
So, this is a topic from Friday,
which is asking us

14
00:00:38,310 --> 00:00:39,900
which of the following
molecules are

15
00:00:39,900 --> 00:00:42,490
free radical species.

16
00:00:42,490 --> 00:00:42,840
OK.

17
00:00:42,840 --> 00:00:45,460
So it looks like most and you
got it, that it's o h.

18
00:00:45,460 --> 00:00:49,560
Let's quickly go over why that
is just in case you weren't

19
00:00:49,560 --> 00:00:52,110
one of that 74%.

20
00:00:52,110 --> 00:00:55,640
So, if we're looking at o h
versus c o, what are we

21
00:00:55,640 --> 00:00:58,890
looking at to decide if we
have a radical here?

22
00:00:58,890 --> 00:01:00,180
Valence electrons.

23
00:01:00,180 --> 00:01:06,130
So we have 6 plus 1 in terms
of o h, that's 7, versus

24
00:01:06,130 --> 00:01:10,020
having 4 plus 6, which is 10.

25
00:01:10,020 --> 00:01:11,800
Specifically, what
do we look for in

26
00:01:11,800 --> 00:01:12,740
terms of valence electrons?

27
00:01:12,740 --> 00:01:14,930
Odd numbers.

28
00:01:14,930 --> 00:01:20,440
So, we have a radical here with
o h, whereas we have an

29
00:01:20,440 --> 00:01:23,280
even number of valence electrons
for c o, so that's

30
00:01:23,280 --> 00:01:26,110
not a radical, in terms of
what we know how to think

31
00:01:26,110 --> 00:01:28,920
about so far, which is using
our Lewis structures to

32
00:01:28,920 --> 00:01:29,990
determine that.

33
00:01:29,990 --> 00:01:30,390
All right.

34
00:01:30,390 --> 00:01:33,600
So that's a little review from
Friday, let's move on to

35
00:01:33,600 --> 00:01:35,570
today's lecture.

36
00:01:35,570 --> 00:01:38,390
In terms of what we're going
to be talking about today,

37
00:01:38,390 --> 00:01:40,650
we're going to finish up with
talking about polar covalent

38
00:01:40,650 --> 00:01:43,480
bonds, and finish up
with discussing the

39
00:01:43,480 --> 00:01:45,880
idea of polar molecules.

40
00:01:45,880 --> 00:01:48,370
And then we'll move on to start
talking about well, we

41
00:01:48,370 --> 00:01:51,140
know how to draw our Lewis
structures, how can we use

42
00:01:51,140 --> 00:01:53,800
those Lewis structures to think
about the actual shapes

43
00:01:53,800 --> 00:01:56,950
of molecules.

44
00:01:56,950 --> 00:02:00,130
So, first thinking about our
polar covalent bonds.

45
00:02:00,130 --> 00:02:03,240
What we had established is if we
have a case where we have a

46
00:02:03,240 --> 00:02:07,440
covalent bond, but there's
unequal sharing of electrons

47
00:02:07,440 --> 00:02:10,890
between the two atoms, namely
because they have different

48
00:02:10,890 --> 00:02:14,660
electronegativities, what we see
is we have a polar bond or

49
00:02:14,660 --> 00:02:17,640
a polar covalent bond where one
of the atoms is going to

50
00:02:17,640 --> 00:02:21,420
pull away more of the electron
density from that bond, giving

51
00:02:21,420 --> 00:02:24,420
it that partial negative charge
where the other atom is

52
00:02:24,420 --> 00:02:27,670
going to end up with a partial
positive charge.

53
00:02:27,670 --> 00:02:30,050
In terms of thinking about
well, what do we call a

54
00:02:30,050 --> 00:02:33,640
covalent bond versus a polar
covalent bond versus a purely

55
00:02:33,640 --> 00:02:36,870
ionic bond, it turns out that
the distinction is a little

56
00:02:36,870 --> 00:02:40,920
bit fuzzy, it's not a clear
line, but in terms of talking

57
00:02:40,920 --> 00:02:43,760
about things in general, and
especially in thinking about

58
00:02:43,760 --> 00:02:46,670
problems for this course, what
we're going to say is if you

59
00:02:46,670 --> 00:02:50,380
have an electronegativity or chi
difference between the two

60
00:02:50,380 --> 00:02:53,350
atoms that is greater than 0 .

61
00:02:53,350 --> 00:02:55,430
4, and less than 1 .

62
00:02:55,430 --> 00:02:58,710
7, and we're talking about the
Pauling electronegativity

63
00:02:58,710 --> 00:03:01,280
scale here, which is what's
used throughout your book.

64
00:03:01,280 --> 00:03:04,010
If you have a difference in
electronegativity anywhere in

65
00:03:04,010 --> 00:03:07,030
that range, we'll have what we
call a polar covalent bond

66
00:03:07,030 --> 00:03:08,650
between those two atoms.

67
00:03:08,650 --> 00:03:12,900
So, for example, if we compare
the chi values for hydrogen

68
00:03:12,900 --> 00:03:15,190
versus carbon, we see
they only have a

69
00:03:15,190 --> 00:03:16,990
difference of 0.4 .

70
00:03:16,990 --> 00:03:19,730
0.4 is not greater than 0.4 .

71
00:03:19,730 --> 00:03:22,910
So we would not call this a c h
bond, we would not say it's

72
00:03:22,910 --> 00:03:26,080
polar covalent, we would
just call it covalent.

73
00:03:26,080 --> 00:03:30,490
In contrast, if we talk about
a carbon oxygen bond, now we

74
00:03:30,490 --> 00:03:34,680
have a chi difference of 0.8 ,
so that is greater than 0.4 ,

75
00:03:34,680 --> 00:03:39,880
so we'll call carbon oxygen
bond polar covalent.

76
00:03:39,880 --> 00:03:42,870
We can extend this idea of
talking about these polar

77
00:03:42,870 --> 00:03:46,330
covalent bonds to thinking
about an entire molecule.

78
00:03:46,330 --> 00:03:49,080
I'm sure you've all heard of
the term a polar molecule

79
00:03:49,080 --> 00:03:52,150
versus a non-polar molecule, and
essentially, when we talk

80
00:03:52,150 --> 00:03:55,450
about having a polar molecule,
what we're saying is that

81
00:03:55,450 --> 00:03:58,030
there's is a net non-zero
dipole moment

82
00:03:58,030 --> 00:03:59,720
within the whole molecule.

83
00:03:59,720 --> 00:04:02,200
So essentially, what we have
to do is combine all of the

84
00:04:02,200 --> 00:04:05,320
individual bonds to think about
the molecule as a whole.

85
00:04:05,320 --> 00:04:10,530
So, let's take an example of
carbon dioxide here, or c o 2.

86
00:04:10,530 --> 00:04:13,130
Very soon in the lecture you'll
learn how to predict

87
00:04:13,130 --> 00:04:15,160
shapes based on Lewis
structures.

88
00:04:15,160 --> 00:04:19,560
So you will quickly see that
this is a linear molecule.

89
00:04:19,560 --> 00:04:21,610
We can think about the
2 bonds within it.

90
00:04:21,610 --> 00:04:25,260
There's two carbon oxygen bonds,
and we know that there

91
00:04:25,260 --> 00:04:27,600
is a dipole in that
bond, which is

92
00:04:27,600 --> 00:04:29,550
going toward the oxygen.

93
00:04:29,550 --> 00:04:31,830
Remember, in chemistry we always
draw the arrow, we're

94
00:04:31,830 --> 00:04:34,000
always interested in what those
electrons are doing.

95
00:04:34,000 --> 00:04:36,560
So if we draw in the bond
dipoles, and you can draw

96
00:04:36,560 --> 00:04:39,980
these into your notes, you want
to draw an arrow towards

97
00:04:39,980 --> 00:04:42,750
the oxygen in each case.

98
00:04:42,750 --> 00:04:45,220
But we're not just interested
here in thinking about do we

99
00:04:45,220 --> 00:04:48,430
have a polar bond, we actually
want to know in general do we

100
00:04:48,430 --> 00:04:49,540
have a polar molecule.

101
00:04:49,540 --> 00:04:53,100
So, looking at c o 2 as a whole,
do you think we have a

102
00:04:53,100 --> 00:04:56,330
polar or a non-polar
molecule here?

103
00:04:56,330 --> 00:04:56,530
Non-polar.

104
00:04:56,530 --> 00:04:59,700
OK, very good for all of you
that said non-polar.

105
00:04:59,700 --> 00:05:02,340
The reason it's non-polar is
we simply have two equal

106
00:05:02,340 --> 00:05:04,870
vectors in opposite directions,
so they cancel

107
00:05:04,870 --> 00:05:05,810
each other out.

108
00:05:05,810 --> 00:05:08,640
The net effect is that
we have a 0 dipole

109
00:05:08,640 --> 00:05:13,360
moment in the c o molecule.

110
00:05:13,360 --> 00:05:16,180
In contrast, we can
look at h 2 o.

111
00:05:16,180 --> 00:05:19,570
H 2 o actually has this bent
shape, and again, we'll see

112
00:05:19,570 --> 00:05:22,860
very soon and how to predict
that h 2 o has a bent shape,

113
00:05:22,860 --> 00:05:24,300
that water is bent.

114
00:05:24,300 --> 00:05:28,490
And again, we do have dipole
moments -- we have bonds where

115
00:05:28,490 --> 00:05:31,310
there is a polar bond and we
should point our arrow toward

116
00:05:31,310 --> 00:05:34,200
the oxygen, because electron
density is being pulled from

117
00:05:34,200 --> 00:05:37,220
the hydrogen atoms
to the oxygen.

118
00:05:37,220 --> 00:05:40,730
And again, we can cancel some
of these vectors out, but we

119
00:05:40,730 --> 00:05:45,200
still have a net dipole moment
that is going to be going up

120
00:05:45,200 --> 00:05:48,660
if we think about combining
these 2 vectors here.

121
00:05:48,660 --> 00:05:52,390
So, what we would say, which
is what we know from

122
00:05:52,390 --> 00:05:54,910
everything we've always
heard, which is that

123
00:05:54,910 --> 00:05:57,040
water is a polar molecule.

124
00:05:57,040 --> 00:05:58,780
So, this is a great
way to think about

125
00:05:58,780 --> 00:06:00,280
molecules in general.

126
00:06:00,280 --> 00:06:05,290
It's very easy to think about
if those vectors cancel each

127
00:06:05,290 --> 00:06:07,870
other out or if they're additive
when we're talking

128
00:06:07,870 --> 00:06:13,810
about just single bonds, so we
have two atoms or if we have

129
00:06:13,810 --> 00:06:17,180
just three atoms, as we do
in these cases here.

130
00:06:17,180 --> 00:06:19,955
But it turns out, a lot of the
molecules we consider are

131
00:06:19,955 --> 00:06:21,960
actually much, much,
much larger than

132
00:06:21,960 --> 00:06:23,690
just a couple of atoms.

133
00:06:23,690 --> 00:06:27,360
So we can't often think about
canceling all the different

134
00:06:27,360 --> 00:06:28,550
vectors out.

135
00:06:28,550 --> 00:06:30,720
And in general, when we're
talking about these really

136
00:06:30,720 --> 00:06:32,820
large molecules, whether
they're large organic

137
00:06:32,820 --> 00:06:35,950
molecules or we're talking about
proteins, which can have

138
00:06:35,950 --> 00:06:39,130
thousands of atoms in them,
instead of adding up all those

139
00:06:39,130 --> 00:06:42,690
individual polar bonds, what
we do is we talk about the

140
00:06:42,690 --> 00:06:45,610
number of polar groups that it
will have in the molecule.

141
00:06:45,610 --> 00:06:48,120
So we can think about a protein,
for example, as

142
00:06:48,120 --> 00:06:51,160
having a lot of different polar
groups or one that has

143
00:06:51,160 --> 00:06:53,410
not too many polar
side groups.

144
00:06:53,410 --> 00:06:56,330
That's one way you hear people
talking about proteins, and

145
00:06:56,330 --> 00:06:59,650
that has a lot to do with their
solubility in water, and

146
00:06:59,650 --> 00:07:02,030
also about how the protein's
going to fold.

147
00:07:02,030 --> 00:07:04,970
But let's look at a little bit
about less complicated example

148
00:07:04,970 --> 00:07:07,440
than a protein with thousands
of different atoms.

149
00:07:07,440 --> 00:07:10,130
Let's instead consider a couple
of vitamins here, which

150
00:07:10,130 --> 00:07:13,090
instead have a few dozen
different atoms in them.

151
00:07:13,090 --> 00:07:17,990
Specifically, let's look at
vitamin A and vitamin B9.

152
00:07:17,990 --> 00:07:26,940
Does anyone know another
name for vitamin B9?

153
00:07:26,940 --> 00:07:30,710
I don't think I hear it -- does
anyone say folic acid.

154
00:07:30,710 --> 00:07:34,220
It's also sometimes called
vitamin M. It's one of the

155
00:07:34,220 --> 00:07:36,860
vitamin B9 vitamins, there's
actually other forms of

156
00:07:36,860 --> 00:07:39,900
vitamin B9, but this is
one of the big ones

157
00:07:39,900 --> 00:07:41,170
that you hear about.

158
00:07:41,170 --> 00:07:43,680
Hopefully you're all very
familiar with folic acid,

159
00:07:43,680 --> 00:07:46,280
because it's an important
vitamin to take, especially if

160
00:07:46,280 --> 00:07:48,530
you're a woman, especially if
there's any chance you ever

161
00:07:48,530 --> 00:07:52,050
might become pregnant in any
kind of near future time, by

162
00:07:52,050 --> 00:07:56,070
accident or on purpose.

163
00:07:56,070 --> 00:07:59,920
And the reason for this is
because folic acid deficiency

164
00:07:59,920 --> 00:08:03,480
in pregnant women leads to
neural tube defects in babies,

165
00:08:03,480 --> 00:08:07,960
and your brain develops very,
very, very early in pregnancy,

166
00:08:07,960 --> 00:08:10,580
the brain of an embryo.

167
00:08:10,580 --> 00:08:13,100
So it turns out that a lot of
women don't realize they're

168
00:08:13,100 --> 00:08:16,870
pregnant while the fetus' brain
is developing, and if

169
00:08:16,870 --> 00:08:19,090
these women are not getting
enough folic acid, you can end

170
00:08:19,090 --> 00:08:21,900
up with spina bifida, which is
an absolutely devastating and

171
00:08:21,900 --> 00:08:25,870
very preventable disease or
other neural tube defects.

172
00:08:25,870 --> 00:08:27,990
So hopefully, if you're not
familiar with folic acid,

173
00:08:27,990 --> 00:08:31,210
you'll become familiar at least
within the next 10 or 15

174
00:08:31,210 --> 00:08:33,990
years or so before you're
thinking about maybe, on

175
00:08:33,990 --> 00:08:36,560
purpose, becoming pregnant
at some time.

176
00:08:36,560 --> 00:08:39,440
Vitamin A you probably all
heard about growing up,

177
00:08:39,440 --> 00:08:41,700
getting enough of your carrots
so you can see at night.

178
00:08:41,700 --> 00:08:44,670
Vitamin A is important
for eye health.

179
00:08:44,670 --> 00:08:46,660
But just looking at the
structure of these two

180
00:08:46,660 --> 00:08:49,390
molecules, knowing what we
know so far about general

181
00:08:49,390 --> 00:08:52,210
chemistry, we can already say
a lot about how these are

182
00:08:52,210 --> 00:08:53,910
going to function the body
or how they'll be

183
00:08:53,910 --> 00:08:55,170
treated in the body.

184
00:08:55,170 --> 00:08:58,310
And specifically, let's take a
look at which of these two

185
00:08:58,310 --> 00:09:01,580
molecules has more polar bonds
and see what that tells us.

186
00:09:01,580 --> 00:09:05,580
So take a look at your two
structures and tell me which

187
00:09:05,580 --> 00:09:10,130
has, between folic acid and
vitamin A, more polar groups

188
00:09:10,130 --> 00:09:19,570
within the vitamin.

189
00:09:19,570 --> 00:09:21,910
And this should go pretty
quickly since you don't need

190
00:09:21,910 --> 00:09:24,800
to have an exact number, we're
just looking and glancing and

191
00:09:24,800 --> 00:09:25,730
seeing which has more.

192
00:09:25,730 --> 00:09:39,300
So let's take 10 seconds
on this.

193
00:09:39,300 --> 00:09:39,780
All right, great.

194
00:09:39,780 --> 00:09:43,260
So most you said folic acid or
vitamin B9, so let's switch

195
00:09:43,260 --> 00:09:46,630
back to our notes and
take a look at why.

196
00:09:46,630 --> 00:09:49,290
So vitamin B9 has more
polar bonds.

197
00:09:49,290 --> 00:09:52,100
It's very easy to see, once I
highlight, that it's not even

198
00:09:52,100 --> 00:09:53,520
a close call at all.

199
00:09:53,520 --> 00:09:57,150
We have a bunch of different
polar bonds in B9 versus just

200
00:09:57,150 --> 00:10:01,300
one in vitamin A. Remember, the
c h bond we're not calling

201
00:10:01,300 --> 00:10:04,080
polar covalent because it only
has an electronegativity

202
00:10:04,080 --> 00:10:07,820
difference of 0.4 between
the two atoms.

203
00:10:07,820 --> 00:10:11,100
So what we know is that vitamin
B9, folic acid, is

204
00:10:11,100 --> 00:10:12,600
more polar.

205
00:10:12,600 --> 00:10:16,370
Remember, we also just said that
water is polar, so would

206
00:10:16,370 --> 00:10:20,700
we say that B9 is water
or fat soluble?

207
00:10:20,700 --> 00:10:23,760
It's going to be water soluble
-- everyone knows the saying,

208
00:10:23,760 --> 00:10:25,980
"like dissolves like."
B9 is going to

209
00:10:25,980 --> 00:10:27,810
be very water soluble.

210
00:10:27,810 --> 00:10:30,240
This actually is very important
if you think about

211
00:10:30,240 --> 00:10:31,250
taking your vitamins.

212
00:10:31,250 --> 00:10:34,220
This means if it's water
soluble, any vitamins that are

213
00:10:34,220 --> 00:10:35,970
water soluble, and now you
should to be able to look at

214
00:10:35,970 --> 00:10:38,780
the structure of any vitamin and
tell me, for example, that

215
00:10:38,780 --> 00:10:41,510
vitamin C is water soluble,
that folic

216
00:10:41,510 --> 00:10:42,760
acid is water soluble.

217
00:10:42,760 --> 00:10:45,090
If you take these vitamins, what
happens is that they're

218
00:10:45,090 --> 00:10:48,340
very quickly and easily excreted
into your urine.

219
00:10:48,340 --> 00:10:51,040
So, for example, some people
like to take mega doses of

220
00:10:51,040 --> 00:10:54,360
vitamin C. What really happens
is that you have a mega dose

221
00:10:54,360 --> 00:10:56,570
of vitamin C in your urine,
it doesn't stick around

222
00:10:56,570 --> 00:10:57,960
long in your body.

223
00:10:57,960 --> 00:11:00,210
So it doesn't help and
just take all of your

224
00:11:00,210 --> 00:11:01,550
vitamin C at once.

225
00:11:01,550 --> 00:11:03,990
That's why it's important to eat
balanced meals throughout

226
00:11:03,990 --> 00:11:06,960
the day, you need to be getting
a constant supply of

227
00:11:06,960 --> 00:11:08,510
these water soluble vitamins.

228
00:11:08,510 --> 00:11:11,000
The same is for folic acid, you
can't just take it once a

229
00:11:11,000 --> 00:11:13,950
month, you need to be taking it
regularly in order that you

230
00:11:13,950 --> 00:11:16,800
keep the stores up in your body,
otherwise you're going

231
00:11:16,800 --> 00:11:19,480
to excrete it, you're going to
get rid of it very quickly.

232
00:11:19,480 --> 00:11:22,490
In contrast, if we think about
vitamin A, is this going to be

233
00:11:22,490 --> 00:11:25,550
water soluble or fat soluble?

234
00:11:25,550 --> 00:11:27,900
Yup, so this is the fat
soluble vitamin.

235
00:11:27,900 --> 00:11:31,440
Vitamin E is another big one
that's fat soluble that gets a

236
00:11:31,440 --> 00:11:36,890
lot of press in terms of being
an important vitamin to take.

237
00:11:36,890 --> 00:11:39,570
We can think about what it means
when a vitamin is fat

238
00:11:39,570 --> 00:11:41,470
soluble instead of
water soluble.

239
00:11:41,470 --> 00:11:43,860
Well, now it's not going to get
excreted out of our body

240
00:11:43,860 --> 00:11:47,020
so quickly, so we actually can
build up amounts of vitamin A

241
00:11:47,020 --> 00:11:50,640
vitamin E, for example, but that
can also pose a problem

242
00:11:50,640 --> 00:11:53,580
if you think about biologically
what's happening.

243
00:11:53,580 --> 00:11:56,110
And up here I just show two
different supplements that I

244
00:11:56,110 --> 00:11:57,020
found on the internet.

245
00:11:57,020 --> 00:12:00,950
This is One A Day vitamin, it
has about 100% of what you

246
00:12:00,950 --> 00:12:02,220
need of everything.

247
00:12:02,220 --> 00:12:05,340
And then this vitamin is one
that I found that's supposed

248
00:12:05,340 --> 00:12:07,790
to really help you with your
eye health, if you have bad

249
00:12:07,790 --> 00:12:10,880
vision instead of glasses, they
suggest that you try this

250
00:12:10,880 --> 00:12:12,060
vitamin here.

251
00:12:12,060 --> 00:12:15,420
And what you can see is that it
has five times your daily

252
00:12:15,420 --> 00:12:19,870
value of vitamin A, and 13 times
what you need of vitamin

253
00:12:19,870 --> 00:12:22,870
E. Is this a good idea?

254
00:12:22,870 --> 00:12:23,580
No.

255
00:12:23,580 --> 00:12:26,670
Basically, you are just building
up more and more and

256
00:12:26,670 --> 00:12:29,890
more of these fat soluble
vitamins into your body.

257
00:12:29,890 --> 00:12:32,070
And there have been studies that
have come out recently

258
00:12:32,070 --> 00:12:34,920
trying to look at the health
benefits of vitamin E, and in

259
00:12:34,920 --> 00:12:37,320
some of these studies they give
these mega doses, and

260
00:12:37,320 --> 00:12:40,120
instead what they find is
increased bleeding in these

261
00:12:40,120 --> 00:12:43,420
patients, an increase in overall
different types of

262
00:12:43,420 --> 00:12:45,530
death that can be happening.

263
00:12:45,530 --> 00:12:48,150
You want to take your vitamins,
it's very important,

264
00:12:48,150 --> 00:12:49,900
but you don't want it just
build up, and you want to

265
00:12:49,900 --> 00:12:52,830
think about is the vitamin that
I'm going and getting 800

266
00:12:52,830 --> 00:12:56,610
times what I need a day, is that
a water soluble vitamin

267
00:12:56,610 --> 00:12:59,145
or a fat soluble vitamin, and
using your chemistry, you

268
00:12:59,145 --> 00:13:01,310
should be able to very quickly
take a quick look at that

269
00:13:01,310 --> 00:13:04,540
structure and figure out what
kind of a vitamin that is.

270
00:13:04,540 --> 00:13:05,200
All right.

271
00:13:05,200 --> 00:13:08,190
So that's just one reason we
would want to be able to think

272
00:13:08,190 --> 00:13:10,920
about polarity is thinking about
whether something is

273
00:13:10,920 --> 00:13:13,790
very water soluble or not.

274
00:13:13,790 --> 00:13:16,740
Let's think about some other
things that have to do with

275
00:13:16,740 --> 00:13:19,390
polarity and then can tell us
a lot of other information.

276
00:13:19,390 --> 00:13:22,670
So let's move on to talking
about the shapes of molecules.

277
00:13:22,670 --> 00:13:26,350
And the shapes of molecules is
very important for a number of

278
00:13:26,350 --> 00:13:28,810
different properties when we're
thinking about chemical

279
00:13:28,810 --> 00:13:32,010
reactions and reactions that
take place in the body.

280
00:13:32,010 --> 00:13:35,090
When we talk about shapes of
molecules, we're talking about

281
00:13:35,090 --> 00:13:37,690
the geometry of that molecule.

282
00:13:37,690 --> 00:13:40,450
And the geometry influences
all of its different

283
00:13:40,450 --> 00:13:43,970
properties, including things
like melting point or boiling

284
00:13:43,970 --> 00:13:45,580
point, it's reactivity.

285
00:13:45,580 --> 00:13:48,640
We just saw when talking about
polar molecules, that it

286
00:13:48,640 --> 00:13:51,380
influences whether or
not the molecule

287
00:13:51,380 --> 00:13:54,440
itself is polar or apolar.

288
00:13:54,440 --> 00:13:57,580
It's also really important when
we talk about biology.

289
00:13:57,580 --> 00:14:00,020
Shape is particularly important
when you think about

290
00:14:00,020 --> 00:14:04,380
enzymes having an active site
where a molecule needs to fit

291
00:14:04,380 --> 00:14:06,875
perfectly into the active
site, it does that

292
00:14:06,875 --> 00:14:08,180
because of its shape.

293
00:14:08,180 --> 00:14:11,190
A quick example that we can
think about is with sucrose.

294
00:14:11,190 --> 00:14:14,610
Does anyone know what
sucrose is?

295
00:14:14,610 --> 00:14:17,780
It's just table sugar -- sucrose
is that crystal in

296
00:14:17,780 --> 00:14:22,870
sugar that we hopefully use in
most of our sugar intake as

297
00:14:22,870 --> 00:14:23,290
sweeteners.

298
00:14:23,290 --> 00:14:26,660
A lot of times we use corn
syrup, which is often not

299
00:14:26,660 --> 00:14:30,350
sucrose and which tends to be
not always as good for us,

300
00:14:30,350 --> 00:14:32,900
although anything in too much
excess is obviously a

301
00:14:32,900 --> 00:14:34,020
non-ideal situation.

302
00:14:34,020 --> 00:14:37,910
But in order for us to use the
energy from sucrose, sucrose

303
00:14:37,910 --> 00:14:40,450
is made up of two individual
sugar monomers.

304
00:14:40,450 --> 00:14:42,620
It's made up of a monomer of
glucose and one of fructose, a

305
00:14:42,620 --> 00:14:46,890
6 and a 5 membered ring, so in
order for our body to use it,

306
00:14:46,890 --> 00:14:49,010
we need to break it into
with its monomers.

307
00:14:49,010 --> 00:14:51,800
And if we that, we can do it by
what's called hydrolysis --

308
00:14:51,800 --> 00:14:54,770
sucrose and water breaks down
into its two monomers, now our

309
00:14:54,770 --> 00:14:56,060
body can use it.

310
00:14:56,060 --> 00:14:58,940
The problem is this process
takes on the order of 10 or

311
00:14:58,940 --> 00:15:02,460
maybe 100 years in order for us
to get enough of the sugar

312
00:15:02,460 --> 00:15:03,520
broken down.

313
00:15:03,520 --> 00:15:06,410
That's why we need an enzyme
molecule, and the enzyme in

314
00:15:06,410 --> 00:15:08,200
our body is called sucrase.

315
00:15:08,200 --> 00:15:12,340
Sucrase breaks down sucrose, it
catalyzes that reaction, so

316
00:15:12,340 --> 00:15:13,720
it happens very quickly.

317
00:15:13,720 --> 00:15:17,460
And the key is, and here's a
ball and stick shape of what

318
00:15:17,460 --> 00:15:18,470
sucrose looks like.

319
00:15:18,470 --> 00:15:21,550
It needs to fit exactly
into that active site.

320
00:15:21,550 --> 00:15:24,750
When it does, it combines into
the enzyme, the enzyme can

321
00:15:24,750 --> 00:15:29,470
then catalyze the hydrolysis or
the breaking apart of those

322
00:15:29,470 --> 00:15:32,800
two individual monomers into
its separate pieces.

323
00:15:32,800 --> 00:15:36,160
And then it let's go of the
glucose and the fructose, and

324
00:15:36,160 --> 00:15:39,050
we get our enzyme back again,
and something else combined

325
00:15:39,050 --> 00:15:40,690
into that active site.

326
00:15:40,690 --> 00:15:44,180
So, this is one very simple
example of why molecular shape

327
00:15:44,180 --> 00:15:45,190
is very important.

328
00:15:45,190 --> 00:15:48,100
A lot of times you're thinking
about small molecules

329
00:15:48,100 --> 00:15:50,410
interacting with proteins or
interacting with other

330
00:15:50,410 --> 00:15:52,580
molecules, and you want to think
about the shape of that

331
00:15:52,580 --> 00:15:55,590
molecule to think about how that
interaction is going to

332
00:15:55,590 --> 00:15:57,740
take place.

333
00:15:57,740 --> 00:16:01,240
So what we're going to use to
think about molecular shape or

334
00:16:01,240 --> 00:16:05,680
molecular geometry is what's
called valence shell electron

335
00:16:05,680 --> 00:16:08,040
repulsion or vsper theory.

336
00:16:08,040 --> 00:16:10,410
And this theory is very
convenient for us to be

337
00:16:10,410 --> 00:16:13,410
thinking about because we are
now masters of drawing Lewis

338
00:16:13,410 --> 00:16:16,680
structures, and vesper theory is
based on Lewis structures,

339
00:16:16,680 --> 00:16:20,450
and also the principle that when
we have valence electron

340
00:16:20,450 --> 00:16:22,950
pairs, they're going to
repel each other.

341
00:16:22,950 --> 00:16:26,220
This make sense any time you
have negatively-charged

342
00:16:26,220 --> 00:16:28,720
electrons, you want to get them
as far away from each

343
00:16:28,720 --> 00:16:29,960
other as possible
because they're

344
00:16:29,960 --> 00:16:32,730
all negatively charged.

345
00:16:32,730 --> 00:16:35,600
And the other principle is that
the geometry around that

346
00:16:35,600 --> 00:16:38,260
central atom, which you've
identified as the central atom

347
00:16:38,260 --> 00:16:40,690
in the Lewis structure, is
going to be such that it

348
00:16:40,690 --> 00:16:44,550
minimizes the repulsion between
those either bonding

349
00:16:44,550 --> 00:16:48,530
electrons or the
electron pairs.

350
00:16:48,530 --> 00:16:50,810
So when we talk about vsper
there's a special nomenclature

351
00:16:50,810 --> 00:16:57,420
that we do use, and in vsper we
have a -- does anyone know

352
00:16:57,420 --> 00:16:59,410
what a means in vsper theory?

353
00:16:59,410 --> 00:17:04,320
It's a central atom.

354
00:17:04,320 --> 00:17:09,030
What about x?

355
00:17:09,030 --> 00:17:11,790
So this is actually the
bonding, any bonding

356
00:17:11,790 --> 00:17:13,320
atom you call x.

357
00:17:13,320 --> 00:17:18,530
And then we have e, which
is equal to lone pairs.

358
00:17:18,530 --> 00:17:21,840
So e is a lone pair -- e is not
a lone pair electron, so

359
00:17:21,840 --> 00:17:28,100
if you have two electrons,
that's one lone pair.

360
00:17:28,100 --> 00:17:28,400
All right.

361
00:17:28,400 --> 00:17:30,750
So there are some guidelines
that we use when we're coming

362
00:17:30,750 --> 00:17:32,350
up with these vsper
geometries.

363
00:17:32,350 --> 00:17:35,490
The first thing we talk about is
the steric number, which we

364
00:17:35,490 --> 00:17:37,880
use to predict what that
geometry will be.

365
00:17:37,880 --> 00:17:40,400
When we're talking about steric
number, all we're

366
00:17:40,400 --> 00:17:44,340
talking about is adding together
the number we have of

367
00:17:44,340 --> 00:17:47,590
bonded atoms, plus the
number of lone pairs.

368
00:17:47,590 --> 00:17:50,580
So, essentially were just adding
x to e and that gives

369
00:17:50,580 --> 00:17:52,440
us our steric number.

370
00:17:52,440 --> 00:17:56,540
So, for example, if we look at
this molecule, which is a x 2

371
00:17:56,540 --> 00:17:59,820
e, what is the steric
number here?

372
00:17:59,820 --> 00:18:03,400
Yeah, it's 3, so we have
a steric number of 3.

373
00:18:03,400 --> 00:18:05,950
Something that I want to point
out is that we could have a

374
00:18:05,950 --> 00:18:10,850
different molecule, which is
also a x 2 e, but in this case

375
00:18:10,850 --> 00:18:14,490
we have a double bond between
the central atom and one of

376
00:18:14,490 --> 00:18:17,950
the x atoms here, and what I
want to point out is in vsper

377
00:18:17,950 --> 00:18:21,680
theory we treat bonds as bonds,
we don't worry about if

378
00:18:21,680 --> 00:18:23,440
they're single or they're double
or they're triple.

379
00:18:23,440 --> 00:18:27,710
So again, we call this a x 2
e, this again, has a steric

380
00:18:27,710 --> 00:18:28,960
number of 3.

381
00:18:28,960 --> 00:18:32,820
So what's important in terms of
vsper theory is the number

382
00:18:32,820 --> 00:18:35,620
of atoms bonded to
a central atom.

383
00:18:35,620 --> 00:18:38,560
What's not important is the
types of bonds that we're

384
00:18:38,560 --> 00:18:41,830
dealing with.

385
00:18:41,830 --> 00:18:44,950
A few other guidelines I want to
mention is first of all, to

386
00:18:44,950 --> 00:18:46,760
think about resonance
structures.

387
00:18:46,760 --> 00:18:49,350
So on Friday we were talking
about the chromate anion, and

388
00:18:49,350 --> 00:18:52,870
we said that here are two of its
resonance structures here,

389
00:18:52,870 --> 00:18:55,030
but that we could actually
draw four more resonance

390
00:18:55,030 --> 00:18:59,320
structures, and I just want to
point out when you take a

391
00:18:59,320 --> 00:19:01,860
molecule that has many different
resonance structures

392
00:19:01,860 --> 00:19:05,730
and you want to draw its vsper,
it's Lewis structure

393
00:19:05,730 --> 00:19:09,940
and its vsper geometry, you
can take any single one of

394
00:19:09,940 --> 00:19:12,340
those resonance structures, it
doesn't matter, you'll all end

395
00:19:12,340 --> 00:19:14,990
up with the same geometry.

396
00:19:14,990 --> 00:19:17,610
And lastly, if we're talking
about a molecule that has more

397
00:19:17,610 --> 00:19:20,420
than one central atom, which is
what we're very, very often

398
00:19:20,420 --> 00:19:22,810
doing, you need to deal with
each one separately.

399
00:19:22,810 --> 00:19:27,480
So for example, with methanol
here, we have a carbon, which

400
00:19:27,480 --> 00:19:30,290
is a central atom, and we also
have an oxygen, which is a

401
00:19:30,290 --> 00:19:33,880
central atom, you need to talk
about the geometry separately,

402
00:19:33,880 --> 00:19:35,330
we don't talk about
the geometry

403
00:19:35,330 --> 00:19:38,840
of the entire molecule.

404
00:19:38,840 --> 00:19:41,740
So, let's go ahead and look at
some of these vsper examples,

405
00:19:41,740 --> 00:19:45,110
and Professor Drennan is going
to help demonstrate what some

406
00:19:45,110 --> 00:19:48,350
of these are with some
models here.

407
00:19:48,350 --> 00:19:50,870
So the first case we're going
to talk about is without any

408
00:19:50,870 --> 00:19:53,830
lone pairs -- this is the most
straightforward case.

409
00:19:53,830 --> 00:19:58,070
And our first case that we can
have is a x 3, which is going

410
00:19:58,070 --> 00:20:02,170
to have a linear shape, and that
will have a bond angle of

411
00:20:02,170 --> 00:20:07,080
a 180 degrees.

412
00:20:07,080 --> 00:20:09,190
The next case that we can
talk about is trigonal

413
00:20:09,190 --> 00:20:11,740
planar or a x 3.

414
00:20:11,740 --> 00:20:14,090
Now, you do need to know
these geometry, you

415
00:20:14,090 --> 00:20:15,130
need to know the names.

416
00:20:15,130 --> 00:20:16,880
A lot of them are very
easy to remember.

417
00:20:16,880 --> 00:20:20,590
I expect not too many of you
will get linear incorrect.

418
00:20:20,590 --> 00:20:22,670
Also, trigonal planar,
pretty easy.

419
00:20:22,670 --> 00:20:26,140
Trigonal, it has 3 atoms around
the central atom and

420
00:20:26,140 --> 00:20:27,240
the molecule is planar.

421
00:20:27,240 --> 00:20:34,850
PROFESSOR: So, what is the bond
angle for this geometry?

422
00:20:34,850 --> 00:20:35,340
120.

423
00:20:35,340 --> 00:20:36,540
PROFESSOR: All right.

424
00:20:36,540 --> 00:20:36,830
Great.

425
00:20:36,830 --> 00:20:41,700
So, next we can think about a x
4, and this is what's called

426
00:20:41,700 --> 00:20:43,510
a tetrahedral geometry.

427
00:20:43,510 --> 00:20:45,870
If you're trying to remember
this name you can think of

428
00:20:45,870 --> 00:20:48,600
each of those bonding atoms
as being in the corner of

429
00:20:48,600 --> 00:20:49,930
tetrahedron.

430
00:20:49,930 --> 00:20:53,230
One thing I want to point out is
you're seeing some notation

431
00:20:53,230 --> 00:20:56,250
we haven't used before in this
class where you have a wedge

432
00:20:56,250 --> 00:20:58,965
coming out at you, and you also
have a dashed line to one

433
00:20:58,965 --> 00:21:00,150
of those bonds.

434
00:21:00,150 --> 00:21:03,350
Any time you see a wedge in a
Lewis structure, it means that

435
00:21:03,350 --> 00:21:05,280
it's coming out at you -- it's
either coming out of the

436
00:21:05,280 --> 00:21:07,620
screen or it's coming
out of your paper.

437
00:21:07,620 --> 00:21:10,670
And any time you see a dashed
line here, which might be

438
00:21:10,670 --> 00:21:13,725
easier to see in your notes,
that means that the bond is

439
00:21:13,725 --> 00:21:17,190
actually going into the page or
into the screen or into the

440
00:21:17,190 --> 00:21:17,470
blackboard.

441
00:21:17,470 --> 00:21:24,140
PROFESSOR: And what are the
angles for a tetrahedral?

442
00:21:24,140 --> 00:21:26,310
PROFESSOR: All right,
so they're 1 0 9 .

443
00:21:26,310 --> 00:21:26,550
5.

444
00:21:26,550 --> 00:21:27,320
Pretty close.

445
00:21:27,320 --> 00:21:30,420
So that's as far away as those
bonds can get from each other

446
00:21:30,420 --> 00:21:31,730
would be 109 .

447
00:21:31,730 --> 00:21:33,910
5 degrees.

448
00:21:33,910 --> 00:21:37,600
The next case we
have is a x 5.

449
00:21:37,600 --> 00:21:40,560
That is what we call trigonal
bipyramidal.

450
00:21:40,560 --> 00:21:43,850
Again, we have trigonal, because
we have those 3 bonds

451
00:21:43,850 --> 00:21:47,700
in the center, and if you can
picture putting walls between

452
00:21:47,700 --> 00:21:50,360
all those bonds, you can see
there's a pyramid on the top

453
00:21:50,360 --> 00:21:52,300
and a pyramid on the bottom,
so bipyramidal.

454
00:21:52,300 --> 00:21:55,820
PROFESSOR: And there are 2 sets
of angles here, what are

455
00:21:55,820 --> 00:22:00,380
the angles for equatorial
atoms?

456
00:22:00,380 --> 00:22:01,230
120.

457
00:22:01,230 --> 00:22:02,220
And then for axial?

458
00:22:02,220 --> 00:22:02,650
90.

459
00:22:02,650 --> 00:22:07,770
PROFESSOR: And again, axial,
those are just the ones in the

460
00:22:07,770 --> 00:22:09,740
axis, and equatorial
if you put your

461
00:22:09,740 --> 00:22:12,360
globe around the equator.

462
00:22:12,360 --> 00:22:15,960
So the last case we
have is a x 6.

463
00:22:15,960 --> 00:22:21,740
A x 6 is what we call octahedral
-- you can picture

464
00:22:21,740 --> 00:22:24,510
each of those bonded atoms as
a corner of an octahedron.

465
00:22:24,510 --> 00:22:27,920
PROFESSOR: And what are
the angles in this,

466
00:22:27,920 --> 00:22:28,810
this one set of angles?

467
00:22:28,810 --> 00:22:30,370
90.

468
00:22:30,370 --> 00:22:30,790
PROFESSOR: Great.

469
00:22:30,790 --> 00:22:32,290
So, 90 degrees.

470
00:22:32,290 --> 00:22:34,090
All right.

471
00:22:34,090 --> 00:22:37,110
So this is all straightforward
when we're just thinking about

472
00:22:37,110 --> 00:22:39,720
different molecular shapes that
don't have any lone pairs

473
00:22:39,720 --> 00:22:42,600
in them, but once we start
having lone pairs, we now need

474
00:22:42,600 --> 00:22:44,570
to think about how those
lone pairs are going

475
00:22:44,570 --> 00:22:47,180
to affect the geometry.

476
00:22:47,180 --> 00:22:50,980
But before we do that, let's
talk about some examples of

477
00:22:50,980 --> 00:22:52,920
molecules without lone pairs.

478
00:22:52,920 --> 00:22:56,340
So the first case is what we saw
at the beginning of class,

479
00:22:56,340 --> 00:22:59,990
carbon dioxide, and we said that
that was linear, and now

480
00:22:59,990 --> 00:23:05,250
we know why it's linear -- it
has a formula of a x 2, and an

481
00:23:05,250 --> 00:23:07,410
s n number of 2.

482
00:23:07,410 --> 00:23:13,250
And that's a bond angle
of 180 degrees.

483
00:23:13,250 --> 00:23:17,850
So, you can tell us about
borane, borane is a x 3.

484
00:23:17,850 --> 00:23:20,420
PROFESSOR: And what
is the geometry?

485
00:23:20,420 --> 00:23:23,910
And the angle?

486
00:23:23,910 --> 00:23:25,910
120.

487
00:23:25,910 --> 00:23:29,600
Feel free to yell out very
loudly, we want people in

488
00:23:29,600 --> 00:23:34,184
OpenCourseWare to hear the
answers coming from the room.

489
00:23:34,184 --> 00:23:34,810
PROFESSOR: All right.

490
00:23:34,810 --> 00:23:37,960
So the next case we're going
to look at is c h 4 or

491
00:23:37,960 --> 00:23:40,690
methane, and let's do a clicker
question here to make

492
00:23:40,690 --> 00:23:43,980
sure everyone is remembering
these geometries.

493
00:23:43,980 --> 00:23:46,640
And this should be very quick,
and try to not turn the page

494
00:23:46,640 --> 00:23:49,380
back one, and tell us in
10 seconds here what

495
00:23:49,380 --> 00:23:50,510
the geometry is.

496
00:23:50,510 --> 00:24:06,480
98%, that is a new record.

497
00:24:06,480 --> 00:24:07,410
Very good.

498
00:24:07,410 --> 00:24:07,620
Tetrahedral.

499
00:24:07,620 --> 00:24:12,420
PROFESSOR: All right, while I'm
holding carbon dioxide in

500
00:24:12,420 --> 00:24:15,540
one hand and methane in the
other hand, I just want to do

501
00:24:15,540 --> 00:24:18,910
an additional plug for the
energy debate tonight.

502
00:24:18,910 --> 00:24:21,980
So there will be representatives
from both

503
00:24:21,980 --> 00:24:24,660
presidential campaigns that
are going to be there to

504
00:24:24,660 --> 00:24:25,670
answer questions.

505
00:24:25,670 --> 00:24:28,870
And I'm not sure of the exact
format, I'm not sure whether

506
00:24:28,870 --> 00:24:31,490
all audience members are
going to be able to ask

507
00:24:31,490 --> 00:24:33,310
questions or not.

508
00:24:33,310 --> 00:24:35,000
I'm not sure if I'm going to be
there, I'm supposed to be

509
00:24:35,000 --> 00:24:38,890
giving a talk actually,
ironically, about energy at

510
00:24:38,890 --> 00:24:40,770
the same time.

511
00:24:40,770 --> 00:24:43,610
But if I can't make it and
you're allowed to ask

512
00:24:43,610 --> 00:24:46,560
questions, I'd like you to
ask a question for me.

513
00:24:46,560 --> 00:24:50,530
So, my lab looks at enzymes that
remove carbon monoxide

514
00:24:50,530 --> 00:24:52,400
and carbon dioxide from
the environment.

515
00:24:52,400 --> 00:24:55,320
And I have an energy initiative
grant to do this,

516
00:24:55,320 --> 00:24:58,750
because MIT recognizes that an
important part of any energy

517
00:24:58,750 --> 00:25:01,910
initiative is thinking about
lowering pollutants and

518
00:25:01,910 --> 00:25:03,630
greenhouse gases.

519
00:25:03,630 --> 00:25:07,040
So, I heard on Thursday at the
debate when Governor Sarah

520
00:25:07,040 --> 00:25:10,140
Palin was asked directly if
she really doesn't think

521
00:25:10,140 --> 00:25:12,570
there's any connection between
global warming and any

522
00:25:12,570 --> 00:25:15,850
man-made activities, that she
said, well, you know, it could

523
00:25:15,850 --> 00:25:19,690
be just regular temperature
fluctuations, and she doesn't

524
00:25:19,690 --> 00:25:21,820
want to point any fingers.

525
00:25:21,820 --> 00:25:23,520
So, she's not quite sure.

526
00:25:23,520 --> 00:25:26,760
I also learned on Thursday that
she will head the energy

527
00:25:26,760 --> 00:25:30,530
initiative in a McCain/Palin
White House.

528
00:25:30,530 --> 00:25:34,180
So, if you go to this debate,
I would like you to ask what

529
00:25:34,180 --> 00:25:37,890
is your plan for sequestering
carbon dioxide and other

530
00:25:37,890 --> 00:25:39,310
greenhouse gases.

531
00:25:39,310 --> 00:25:41,840
You can ask both candidates,
and please let me know what

532
00:25:41,840 --> 00:25:44,400
the answer is, because I think
that's a really important

533
00:25:44,400 --> 00:25:46,600
question -- here we have carbon
dioxide, here we have

534
00:25:46,600 --> 00:25:48,200
methane, we need to be thinking

535
00:25:48,200 --> 00:25:50,930
about greenhouse gases.

536
00:25:50,930 --> 00:25:52,810
PROFESSOR: You can also throw
in the question that you do

537
00:25:52,810 --> 00:25:54,170
know this is tetrahedral
and 109 .

538
00:25:54,170 --> 00:26:00,760
5 degrees -- show your chemistry
knowledge here.

539
00:26:00,760 --> 00:26:02,990
All right, and let's keep
showing that we know our

540
00:26:02,990 --> 00:26:08,300
geometries, let's
look at p c l 5.

541
00:26:08,300 --> 00:26:12,240
PROFESSOR: And what
is geometry here?

542
00:26:12,240 --> 00:26:12,820
Yup.

543
00:26:12,820 --> 00:26:15,960
And the angles?

544
00:26:15,960 --> 00:26:18,700
120 and 90.

545
00:26:18,700 --> 00:26:22,520
PROFESSOR: So, 120 equatorial,
and 90 axial.

546
00:26:22,520 --> 00:26:28,450
And last we have s f 6.

547
00:26:28,450 --> 00:26:29,990
PROFESSOR: So, what is
the geometry here?

548
00:26:29,990 --> 00:26:31,040
Octahedral.

549
00:26:31,040 --> 00:26:31,370
And angle?

550
00:26:31,370 --> 00:26:31,550
90.

551
00:26:31,550 --> 00:26:37,270
PROFESSOR: All right, so there's
our set of examples,

552
00:26:37,270 --> 00:26:41,020
one for each for those shapes
or geometries that have no

553
00:26:41,020 --> 00:26:42,240
lone pairs.

554
00:26:42,240 --> 00:26:44,600
And now let's think a little
bit about once we do have

555
00:26:44,600 --> 00:26:46,820
molecules with lone pairs.

556
00:26:46,820 --> 00:26:49,840
And the biggest point to keep
in mind when we're comparing

557
00:26:49,840 --> 00:26:53,580
lone pair electrons with bonding
electrons or electrons

558
00:26:53,580 --> 00:26:56,850
in bonds, is that when you have
electrons in bonds they

559
00:26:56,850 --> 00:27:00,060
have less spatial distribution
than lone pairs.

560
00:27:00,060 --> 00:27:03,340
So that's just a way of saying
that electrons and bonds, they

561
00:27:03,340 --> 00:27:05,930
take up less space.

562
00:27:05,930 --> 00:27:09,060
So, when we think about lone
pair electrons, they're taking

563
00:27:09,060 --> 00:27:11,830
up more space and this means
that they're going to

564
00:27:11,830 --> 00:27:15,660
experience more repulsion in
thinking about the lone pair

565
00:27:15,660 --> 00:27:19,220
electrons with either other lone
pair electrons or with

566
00:27:19,220 --> 00:27:20,310
bonding electrons.

567
00:27:20,310 --> 00:27:22,400
So let's think a second
about the order that

568
00:27:22,400 --> 00:27:23,520
this will be in.

569
00:27:23,520 --> 00:27:26,240
The biggest repulsion we would
feel is if we have two

570
00:27:26,240 --> 00:27:29,640
different lone pair electrons
or lone pairs.

571
00:27:29,640 --> 00:27:31,660
They're going to have
the most repulsion.

572
00:27:31,660 --> 00:27:34,700
In the middle is if we're
talking about a lone pair with

573
00:27:34,700 --> 00:27:36,050
a bonding pair.

574
00:27:36,050 --> 00:27:38,640
And then the least repulsion
is going to be between two

575
00:27:38,640 --> 00:27:43,870
bonds or two bonding
pairs of electrons.

576
00:27:43,870 --> 00:27:46,650
So, let's look at an example
of where this comes up.

577
00:27:46,650 --> 00:27:49,780
So the first example I'm going
to talk about is a molecule

578
00:27:49,780 --> 00:27:53,810
that has the geometry of a
seesaw shape, and once we get

579
00:27:53,810 --> 00:27:56,160
to having Professor Drennan
actually show you that shape,

580
00:27:56,160 --> 00:27:57,480
you will never forget.

581
00:27:57,480 --> 00:27:59,240
See-saw, that's going to
be one of the easy

582
00:27:59,240 --> 00:28:00,790
geometries to remember.

583
00:28:00,790 --> 00:28:03,350
But the first thing that we need
to consider in terms of

584
00:28:03,350 --> 00:28:08,380
the shape, which starts, if you
picture first of all, and

585
00:28:08,380 --> 00:28:11,380
these models sometimes don't
quite stay together, we could

586
00:28:11,380 --> 00:28:13,400
actually have two different
possibilities.

587
00:28:13,400 --> 00:28:16,790
The first is the idea that we
could have an axial lone pair,

588
00:28:16,790 --> 00:28:19,520
and the second is the
possibility that we could have

589
00:28:19,520 --> 00:28:21,370
an equatorial lone pair.

590
00:28:21,370 --> 00:28:23,980
And we could consider both, and
we should be able to use

591
00:28:23,980 --> 00:28:26,710
our vsper principles to think
about which one is actually

592
00:28:26,710 --> 00:28:27,860
going to happen.

593
00:28:27,860 --> 00:28:31,710
So if we think about having an
axial lone pair here, that

594
00:28:31,710 --> 00:28:34,210
would mean that these lone pair
electrons are going to be

595
00:28:34,210 --> 00:28:38,030
within 90 degrees of three
different loan pairs, and

596
00:28:38,030 --> 00:28:40,693
actually we'll see later, it
will be more than 90 because

597
00:28:40,693 --> 00:28:42,220
they're actually going
to push them down.

598
00:28:42,220 --> 00:28:44,990
But in terms of considering
how many bonding electron

599
00:28:44,990 --> 00:28:49,090
pairs they'll repel strongly,
what we care about is anything

600
00:28:49,090 --> 00:28:51,040
within 90 degrees initially.

601
00:28:51,040 --> 00:28:53,850
So, what we would see with the
axial lone pair is that we

602
00:28:53,850 --> 00:28:56,460
have three lone pairs
that we're going to

603
00:28:56,460 --> 00:28:59,040
very strongly repel.

604
00:28:59,040 --> 00:29:01,940
Now would you tell me, if we
think about an equatorial lone

605
00:29:01,940 --> 00:29:07,120
pair, how many different bonding
electrons or a bonding

606
00:29:07,120 --> 00:29:10,570
pairs that will strongly
repel if we have an

607
00:29:10,570 --> 00:29:11,750
equatorial lone pair?

608
00:29:11,750 --> 00:29:15,280
So you might need to look at
your notes to actually compare

609
00:29:15,280 --> 00:29:31,040
these two before you submit
your answer up here.

610
00:29:31,040 --> 00:29:31,270
All right.

611
00:29:31,270 --> 00:29:45,010
Let's take 10 seconds on that.

612
00:29:45,010 --> 00:29:47,540
OK, strong showing with the
clicker questions today.

613
00:29:47,540 --> 00:29:51,450
It's going to be a tight race
for clicker competition.

614
00:29:51,450 --> 00:29:53,730
It's correct that the equatorial
is, in fact, has

615
00:29:53,730 --> 00:29:58,330
two, only two that it
strongly repels.

616
00:29:58,330 --> 00:30:00,860
So, actually it's probably
easier to look at Professor

617
00:30:00,860 --> 00:30:02,920
Drennan's model here
to see that.

618
00:30:02,920 --> 00:30:06,980
So it's where the equatorial
lone pair is that is the

619
00:30:06,980 --> 00:30:10,320
actual seesaw geometry.

620
00:30:10,320 --> 00:30:13,610
PROFESSOR: So, how many of you
had seesaws in playgrounds

621
00:30:13,610 --> 00:30:15,650
when you were growing up?

622
00:30:15,650 --> 00:30:16,290
Oh, a lot of people.

623
00:30:16,290 --> 00:30:18,300
I know that they're not
considered totally safe

624
00:30:18,300 --> 00:30:20,800
anymore, so some of them
are going away.

625
00:30:20,800 --> 00:30:25,120
So this is seesaw.

626
00:30:25,120 --> 00:30:32,660
Now you should never
forget it.

627
00:30:32,660 --> 00:30:35,100
PROFESSOR: All right, so seesaw,
we've got seesaw.

628
00:30:35,100 --> 00:30:38,470
Just to point out a
few more shapes.

629
00:30:38,470 --> 00:30:42,750
We took one out, we replaced one
bond with a lone pair in

630
00:30:42,750 --> 00:30:43,190
terms of seesaw.

631
00:30:43,190 --> 00:30:46,650
If we have a x 3 e
2 now we have two

632
00:30:46,650 --> 00:30:49,190
equatorial lone pairs.

633
00:30:49,190 --> 00:30:50,210
This is called the T-shaped

634
00:30:50,210 --> 00:30:51,220
PROFESSOR: T-shaped.

635
00:30:51,220 --> 00:30:55,290
PROFESSOR: See, these aren't
too hard to remember the

636
00:30:55,290 --> 00:30:58,160
geometries, the geometry
names.

637
00:30:58,160 --> 00:31:02,920
And we also can think about
if we have a x 4 e 2.

638
00:31:02,920 --> 00:31:06,030
So now in order to get our lone
pairs as far away from

639
00:31:06,030 --> 00:31:08,340
each other as possible, they're
going to be on the

640
00:31:08,340 --> 00:31:15,760
axial position, and this is
called square planer.

641
00:31:15,760 --> 00:31:18,190
PROFESSOR: So, this is easy to
remember, square, also planar.

642
00:31:18,190 --> 00:31:22,530
PROFESSOR: All right.

643
00:31:22,530 --> 00:31:24,900
So, let's talk a little bit
now about what the actual

644
00:31:24,900 --> 00:31:27,950
angles are between bonds
when now we have

645
00:31:27,950 --> 00:31:29,710
these lone pairs present.

646
00:31:29,710 --> 00:31:33,540
And remember, what we said is
when we have a lone pair of

647
00:31:33,540 --> 00:31:36,390
electrons, they actually are
going to have more repulsion

648
00:31:36,390 --> 00:31:40,160
than if we just have
a c h bond or a

649
00:31:40,160 --> 00:31:41,990
bonded pair of a electrons.

650
00:31:41,990 --> 00:31:46,230
So essentially what that means
is that in molecules that have

651
00:31:46,230 --> 00:31:49,195
lone pair electrons, so for
example, if we look at n h 3

652
00:31:49,195 --> 00:31:53,580
or ammonia versus methane here,
and Professor Drennan is

653
00:31:53,580 --> 00:31:55,900
showing those models
to you here.

654
00:31:55,900 --> 00:31:59,570
What you end up seeing is
actually that the bonding

655
00:31:59,570 --> 00:32:04,400
angle in n h 3 between the
n h bonds is smaller.

656
00:32:04,400 --> 00:32:08,470
And you can't see it with these
models because these are

657
00:32:08,470 --> 00:32:10,530
not actual lone pairs.

658
00:32:10,530 --> 00:32:12,580
But if they were actually
repelling each other, they

659
00:32:12,580 --> 00:32:16,510
would be pushing those bonds as
far away as possible, and

660
00:32:16,510 --> 00:32:18,340
instead of being 109 .

661
00:32:18,340 --> 00:32:21,680
5, you'd see that the
angle is now 106 .

662
00:32:21,680 --> 00:32:22,290
7.

663
00:32:22,290 --> 00:32:24,680
So it's a smaller angle between
bonds, because you

664
00:32:24,680 --> 00:32:26,580
have more repulsion from
those lone pairs

665
00:32:26,580 --> 00:32:29,830
pushing those bonds down.

666
00:32:29,830 --> 00:32:32,860
We can also think about the
influence of atomic size in

667
00:32:32,860 --> 00:32:34,390
terms of this effect.

668
00:32:34,390 --> 00:32:37,560
So first of all, what happens to
atomic size as you go down

669
00:32:37,560 --> 00:32:39,680
the periodic table?

670
00:32:39,680 --> 00:32:41,950
Good, it increases.

671
00:32:41,950 --> 00:32:45,830
So we have size increasing as we
go down the periodic table.

672
00:32:45,830 --> 00:32:48,370
Phosphorous is right underneath
nitrogen on the

673
00:32:48,370 --> 00:32:50,370
periodic table, so phosphorous
is going to

674
00:32:50,370 --> 00:32:51,900
be bigger than nitrogen.

675
00:32:51,900 --> 00:32:54,900
In terms of picturing what
happens with those lone pairs,

676
00:32:54,900 --> 00:32:57,690
when we have a larger atom, the
orbitals are also going to

677
00:32:57,690 --> 00:33:00,290
be larger, they can take
up more space.

678
00:33:00,290 --> 00:33:04,370
That means these electrons, this
lone pair electrons are

679
00:33:04,370 --> 00:33:06,240
going to take up more space.

680
00:33:06,240 --> 00:33:09,800
This means they're going to
push away those bonding

681
00:33:09,800 --> 00:33:11,580
electrons even more.

682
00:33:11,580 --> 00:33:14,150
So would you expect the
bond to be larger or

683
00:33:14,150 --> 00:33:16,040
smaller for p h 3?

684
00:33:16,040 --> 00:33:17,300
STUDENT: [INAUDIBLE]

685
00:33:17,300 --> 00:33:18,850
PROFESSOR: It's going
to be smaller.

686
00:33:18,850 --> 00:33:22,130
So, what we see is that angles
between bonded atoms actually

687
00:33:22,130 --> 00:33:25,990
decrease as you go down a column
on the periodic table.

688
00:33:25,990 --> 00:33:29,910
So the actual angles for p h 3
are now going to be really

689
00:33:29,910 --> 00:33:32,320
quite a bit smaller,
they're 93 .

690
00:33:32,320 --> 00:33:38,130
3 degrees.

691
00:33:38,130 --> 00:33:38,430
All right.

692
00:33:38,430 --> 00:33:42,140
So let's create a list for
ourselves in terms of all the

693
00:33:42,140 --> 00:33:45,300
geometries that we can have now
that we're dealing with

694
00:33:45,300 --> 00:33:46,950
lone pairs.

695
00:33:46,950 --> 00:33:49,740
One thing I want to point out
in terms of remembering the

696
00:33:49,740 --> 00:33:52,410
names of the different
geometries, when you're naming

697
00:33:52,410 --> 00:33:55,690
a geometry, the geometry name,
for example, when we looked at

698
00:33:55,690 --> 00:33:59,910
square planar, we're only
actually naming where the

699
00:33:59,910 --> 00:34:02,400
bonds are, where actual
atoms are.

700
00:34:02,400 --> 00:34:05,020
The name doesn't really depend
on the lone pairs.

701
00:34:05,020 --> 00:34:07,810
And also, when you draw a
geometry, you don't always

702
00:34:07,810 --> 00:34:10,710
have to draw the lone pairs in,
but you have to remember

703
00:34:10,710 --> 00:34:13,910
that the lone pairs are very
much affecting the angles

704
00:34:13,910 --> 00:34:17,020
within your molecule and
also the actual shape.

705
00:34:17,020 --> 00:34:20,310
So, for example, let's start
talking about different types

706
00:34:20,310 --> 00:34:22,120
that have lone pairs in it.

707
00:34:22,120 --> 00:34:26,530
First of all, thinking about a
x 2 e, so we have one lone

708
00:34:26,530 --> 00:34:31,530
pair, and the geometry here is
bent, and I want you, thinking

709
00:34:31,530 --> 00:34:35,160
about lone pair repulsion, to
tell us what you think the

710
00:34:35,160 --> 00:34:44,160
angle between these bonds
are going to be?

711
00:34:44,160 --> 00:34:59,040
And let's take 10
seconds on that.

712
00:34:59,040 --> 00:34:59,650
OK, good.

713
00:34:59,650 --> 00:35:01,940
75%, that's not bad.

714
00:35:01,940 --> 00:35:07,920
Let's think about why.

715
00:35:07,920 --> 00:35:11,830
It's going to be less than 120
degrees, because we know that

716
00:35:11,830 --> 00:35:16,550
normally in a trigonal planar
situation, we have an angle of

717
00:35:16,550 --> 00:35:17,890
120 degrees.

718
00:35:17,890 --> 00:35:21,520
And since lone pairs are going
to cause more repulsion, we're

719
00:35:21,520 --> 00:35:25,060
actually pushing down these
two bonds here closer

720
00:35:25,060 --> 00:35:27,840
together, so what we end up
seeing is that they're going

721
00:35:27,840 --> 00:35:30,580
to be less than 120 degrees.

722
00:35:30,580 --> 00:35:35,110
And it depends on the actual
molecule what the exact angles

723
00:35:35,110 --> 00:35:37,570
are going to be, so you never
have to learn the exact angles

724
00:35:37,570 --> 00:35:39,940
in terms of lone pair electrons,
you just need to be

725
00:35:39,940 --> 00:35:42,170
able to tell us if the bond
angle is going to be less than

726
00:35:42,170 --> 00:35:44,870
120 degrees.

727
00:35:44,870 --> 00:35:49,250
So let's look at another example
here, a x 3 with one

728
00:35:49,250 --> 00:35:50,920
lone pair, e.

729
00:35:50,920 --> 00:35:53,170
This is called trigonal
pyramidal.

730
00:35:53,170 --> 00:35:56,810
Again, you have trigonal because
there's three atoms

731
00:35:56,810 --> 00:35:58,800
bonded to the central
atom, and this looks

732
00:35:58,800 --> 00:36:02,050
like a pyramid here.

733
00:36:02,050 --> 00:36:03,660
PROFESSOR: So, what would
the angle of this be?

734
00:36:03,660 --> 00:36:04,326
STUDENT: [INAUDIBLE]

735
00:36:04,326 --> 00:36:09,680
PROFESSOR: I think I heard it.

736
00:36:09,680 --> 00:36:10,860
PROFESSOR: I think so, too.

737
00:36:10,860 --> 00:36:13,290
Less than 109 .

738
00:36:13,290 --> 00:36:14,960
5.

739
00:36:14,960 --> 00:36:18,500
Let's take another example,
a x 2 e 2.

740
00:36:18,500 --> 00:36:19,560
This is also bent.

741
00:36:19,560 --> 00:36:22,280
Tell us what the geometry -- we
told you the geometry, tell

742
00:36:22,280 --> 00:36:24,530
us what the bond angle
is going to be

743
00:36:24,530 --> 00:36:25,780
between these bonds.

744
00:36:25,780 --> 00:36:30,780
So, let's take 10
seconds on this.

745
00:36:30,780 --> 00:36:38,780
Let's re-poll F 4.

746
00:36:38,780 --> 00:36:52,690
So 10 seconds again.

747
00:36:52,690 --> 00:36:53,620
All right.

748
00:36:53,620 --> 00:36:57,230
In about 10 seconds, Darcy,
in 10 seconds we'll

749
00:36:57,230 --> 00:36:59,310
hit the next side.

750
00:36:59,310 --> 00:37:09,130
OK, so it's less than 109 .

751
00:37:09,130 --> 00:37:10,910
5 degrees now.

752
00:37:10,910 --> 00:37:15,680
So some of you wrote that it was
less than 120 degrees, and

753
00:37:15,680 --> 00:37:18,730
we can think about if we switch
back to the class notes

754
00:37:18,730 --> 00:37:20,700
the difference here.

755
00:37:20,700 --> 00:37:24,330
So even though, so this is now
going to be less than 109 .

756
00:37:24,330 --> 00:37:28,680
5, if we looked at the case we
had bent in the first case,

757
00:37:28,680 --> 00:37:31,920
that started with 120 and one
of those bonds was replaced

758
00:37:31,920 --> 00:37:35,730
with a lone pair, but now we
have two lone pairs here, so

759
00:37:35,730 --> 00:37:38,450
now what we're going to see is
that the two bents are not, in

760
00:37:38,450 --> 00:37:39,090
fact, equal.

761
00:37:39,090 --> 00:37:42,540
The bent where you started with
the tetrahedral shape is

762
00:37:42,540 --> 00:37:44,530
actually going to be
less than 109 .

763
00:37:44,530 --> 00:37:48,660
5, and Professor Drennan can
maybe show us that with the

764
00:37:48,660 --> 00:37:49,590
models here.

765
00:37:49,590 --> 00:37:52,340
PROFESSOR: It's a little had to
see, actually, between the

766
00:37:52,340 --> 00:37:59,826
two, but I think it's easier
to see on this, that if you

767
00:37:59,826 --> 00:38:03,970
consider the sort of starting
place for the two, if you look

768
00:38:03,970 --> 00:38:07,900
at the bottom set and you put
orbitals, so it also depends

769
00:38:07,900 --> 00:38:09,410
on your starting geometry.

770
00:38:09,410 --> 00:38:15,470
It can be bent if take off these
bonds on the top, but it

771
00:38:15,470 --> 00:38:18,890
depends on what the starting
geometries were.

772
00:38:18,890 --> 00:38:20,970
And so you will go with your
starting geometry,

773
00:38:20,970 --> 00:38:22,200
if it was 109 .

774
00:38:22,200 --> 00:38:23,670
5, it'll be less than that.

775
00:38:23,670 --> 00:38:26,450
If it's 120, then it's
less than that.

776
00:38:26,450 --> 00:38:28,050
PROFESSOR: So, this is a good
case of where, even when you

777
00:38:28,050 --> 00:38:30,310
have the geometry the same, you
need to think about how

778
00:38:30,310 --> 00:38:32,700
many lone pairs you're dealing
with in your molecule, and

779
00:38:32,700 --> 00:38:35,530
think about what would the
geometry be if I just pictured

780
00:38:35,530 --> 00:38:38,480
those lone pairs in there as
bonds and then push them even

781
00:38:38,480 --> 00:38:41,550
closer together so that it's
less than that actual angle.

782
00:38:41,550 --> 00:38:48,340
OK, let's talk about a x 4 e,
and that is what we call the

783
00:38:48,340 --> 00:38:49,450
seesaw shape.

784
00:38:49,450 --> 00:38:54,620
PROFESSOR: And so, what
difference in angles are you

785
00:38:54,620 --> 00:38:55,060
going to have?

786
00:38:55,060 --> 00:38:58,100
STUDENT: [INAUDIBLE]

787
00:38:58,100 --> 00:38:58,390
PROFESSOR: Yup.

788
00:38:58,390 --> 00:39:02,640
PROFESSOR 2: So, two sets again,
one is less than 120,

789
00:39:02,640 --> 00:39:05,630
and one is going to
be less than 90.

790
00:39:05,630 --> 00:39:09,650
So, let's look at a x 3 e 2,
you'll notice there's a lot

791
00:39:09,650 --> 00:39:12,140
more combinations we can get
to once we're talking about

792
00:39:12,140 --> 00:39:13,630
lone pairs.

793
00:39:13,630 --> 00:39:17,460
This is called T-shaped, and we
had briefly discussed that

794
00:39:17,460 --> 00:39:21,270
but not mention what the
angles would be.

795
00:39:21,270 --> 00:39:22,730
PROFESSOR: So, what angle
would you have here?

796
00:39:22,730 --> 00:39:25,240
Yup.

797
00:39:25,240 --> 00:39:26,300
PROFESSOR: All right, great.

798
00:39:26,300 --> 00:39:28,320
Less than 90 degrees.

799
00:39:28,320 --> 00:39:34,300
So, we'll start a new page here
and talk about a x 2 e 3,

800
00:39:34,300 --> 00:39:41,270
and we'll let you tell us
what this geometry is.

801
00:39:41,270 --> 00:39:44,420
Yup, so I hear a lot of
linear out there.

802
00:39:44,420 --> 00:39:47,460
And you need to keep in mind
we give the names based on

803
00:39:47,460 --> 00:39:50,430
where the actual bonds are, not
where the lone pairs are,

804
00:39:50,430 --> 00:39:52,840
even though the lone pairs, of
course, affect the structure

805
00:39:52,840 --> 00:39:54,470
in the geometry.

806
00:39:54,470 --> 00:39:56,110
PROFESSOR: The angle?

807
00:39:56,110 --> 00:39:58,290
STUDENT: 180.

808
00:39:58,290 --> 00:39:58,860
PROFESSOR: Yup, 180.

809
00:39:58,860 --> 00:40:02,170
PROFESSOR: So, it's exactly
180 here, not less than.

810
00:40:02,170 --> 00:40:08,850
PROFESSOR: So, for a x 5 e, what
we have here is called

811
00:40:08,850 --> 00:40:09,620
square pyramidal.

812
00:40:09,620 --> 00:40:13,790
This is square because of the
four square at the bottom of

813
00:40:13,790 --> 00:40:16,310
your pyramid, then you can
picture the pyramid as you go

814
00:40:16,310 --> 00:40:17,080
to the top.

815
00:40:17,080 --> 00:40:21,990
PROFESSOR: And so what angles
would you have here?

816
00:40:21,990 --> 00:40:22,170
Yup, less than 90.

817
00:40:22,170 --> 00:40:26,230
PROFESSOR: So, less
than 90, great.

818
00:40:26,230 --> 00:40:32,770
All right, there are more.

819
00:40:32,770 --> 00:40:35,430
So, our next combination that
we can think about is if we

820
00:40:35,430 --> 00:40:40,800
have four bonded atoms and two
lone pairs, a x 4 e 2, that's

821
00:40:40,800 --> 00:40:42,710
going to be called
square planar.

822
00:40:42,710 --> 00:40:47,490
PROFESSOR: So, again, we have
square and it's planar.

823
00:40:47,490 --> 00:40:49,870
And so what are your
angles here?

824
00:40:49,870 --> 00:40:51,340
Yup, exactly 90.

825
00:40:51,340 --> 00:40:58,350
PROFESSOR: So next let's
look at a x 3 e 3.

826
00:40:58,350 --> 00:41:01,660
This is what we call T-shape,
this is also T-shape.

827
00:41:01,660 --> 00:41:08,560
PROFESSOR: And angle here?

828
00:41:08,560 --> 00:41:13,260
PROFESSOR: Yup, I think we heard
less than, it's less

829
00:41:13,260 --> 00:41:14,750
than 90 degrees.

830
00:41:14,750 --> 00:41:20,510
And let's look at a x 2 e 4, and
again, you guys can tell

831
00:41:20,510 --> 00:41:24,230
us what this geometry
is, first of all.

832
00:41:24,230 --> 00:41:25,220
Yup, it's linear.

833
00:41:25,220 --> 00:41:28,542
So, we have lots of different
ways we can get to a linear

834
00:41:28,542 --> 00:41:29,820
molecule, some of which have
lots of lone pairs, some of

835
00:41:29,820 --> 00:41:31,880
which have no lone
pairs at all.

836
00:41:31,880 --> 00:41:37,490
This is linear and
a 180 degrees.

837
00:41:37,490 --> 00:41:37,740
All right.

838
00:41:37,740 --> 00:41:40,770
So we'll do just a few examples
to show you some

839
00:41:40,770 --> 00:41:42,890
actual molecules that have
these geometries.

840
00:41:42,890 --> 00:41:45,540
We won't go through all of
them, because as we saw,

841
00:41:45,540 --> 00:41:47,670
there's lots of different
combinations we can have once

842
00:41:47,670 --> 00:41:49,910
we start getting into
lone pairs.

843
00:41:49,910 --> 00:41:53,800
The first we'll look at is water
here, and water is a

844
00:41:53,800 --> 00:41:57,320
good one to look at because we
had seen that at the beginning

845
00:41:57,320 --> 00:41:58,560
of class here when
we were talking

846
00:41:58,560 --> 00:42:00,800
about all polar molecules.

847
00:42:00,800 --> 00:42:03,860
When we talk about the formula
type of water, what when you

848
00:42:03,860 --> 00:42:07,330
say the formula type is?

849
00:42:07,330 --> 00:42:09,050
STUDENT: [INAUDIBLE]

850
00:42:09,050 --> 00:42:11,350
PROFESSOR: A x 2 e 2,
that's correct.

851
00:42:11,350 --> 00:42:14,400
And what is the geometry
of water?

852
00:42:14,400 --> 00:42:16,250
It's bent.

853
00:42:16,250 --> 00:42:19,170
So what we saw at the beginning
of class is water is

854
00:42:19,170 --> 00:42:21,550
bent and now you can see why and
should be able to predict

855
00:42:21,550 --> 00:42:22,570
that yourself.

856
00:42:22,570 --> 00:42:25,200
I just want to mention that if
you look, and depending on the

857
00:42:25,200 --> 00:42:28,250
edition of the book you have,
in one edition it's called

858
00:42:28,250 --> 00:42:30,530
bent, in the other edition
it's called angular.

859
00:42:30,530 --> 00:42:33,590
We'll call it bent, but just
remember that bent and

860
00:42:33,590 --> 00:42:37,190
angular, they're the
same geometry.

861
00:42:37,190 --> 00:42:41,200
Let's look at another
example, s f 4.

862
00:42:41,200 --> 00:42:43,370
And here it is drawn here.

863
00:42:43,370 --> 00:42:49,560
What is the formula
type for s f 4?

864
00:42:49,560 --> 00:42:52,540
I think I heard it, a x 4 e.

865
00:42:52,540 --> 00:42:55,230
And the geometry?

866
00:42:55,230 --> 00:42:56,720
Seesaw, good.

867
00:42:56,720 --> 00:42:59,510
All right, we never get seesaw
wrong, that's the easy

868
00:42:59,510 --> 00:43:02,360
give-away one.

869
00:43:02,360 --> 00:43:05,870
All right, what about b r f 3?

870
00:43:05,870 --> 00:43:08,620
This is the shape here.

871
00:43:08,620 --> 00:43:13,560
What is the geometry for b f 3
-- the original geometry, not

872
00:43:13,560 --> 00:43:17,230
the new geometry here?

873
00:43:17,230 --> 00:43:18,210
What was that?

874
00:43:18,210 --> 00:43:19,620
STUDENT: T.

875
00:43:19,620 --> 00:43:21,300
PROFESSOR: T-shaped,
that's right.

876
00:43:21,300 --> 00:43:30,930
B f 3 is T-shaped.

877
00:43:30,930 --> 00:43:35,620
All right, so let's
look at xenon f 2.

878
00:43:35,620 --> 00:43:39,890
That's going to have a formula
type of a x 2 e 3, if we

879
00:43:39,890 --> 00:43:43,520
actually look at the Lewis
structure here.

880
00:43:43,520 --> 00:43:47,090
So if you think about what the
geometry is, and you should

881
00:43:47,090 --> 00:43:49,460
just be able to look at this
and see, what would you say

882
00:43:49,460 --> 00:43:52,350
the geometry of xenon f 2 is?

883
00:43:52,350 --> 00:43:53,950
STUDENT: Linear.

884
00:43:53,950 --> 00:43:55,230
PROFESSOR: Linear, good.

885
00:43:55,230 --> 00:43:55,710
All right.

886
00:43:55,710 --> 00:44:01,690
And let's try one more here,
which is a x 4 e 2, or a xenon

887
00:44:01,690 --> 00:44:06,110
f 4 -- very explosive,
but a good example.

888
00:44:06,110 --> 00:44:08,050
STUDENT: Square planar.

889
00:44:08,050 --> 00:44:10,000
PROFESSOR: Square planar,
all right, great.

890
00:44:10,000 --> 00:44:14,550
So, in general, when we think
about vsper theory, even

891
00:44:14,550 --> 00:44:17,430
though it doesn't talk about or
tell us anything about the

892
00:44:17,430 --> 00:44:20,420
energies of these different
shapes, it's very useful in

893
00:44:20,420 --> 00:44:22,990
making a first approximation
and coming very close to

894
00:44:22,990 --> 00:44:26,810
thinking about what the actual
shapes of molecules are.

895
00:44:26,810 --> 00:44:28,810
So one thing I want to point
out in terms of what you're

896
00:44:28,810 --> 00:44:32,660
responsible for is you should
be able to fill out one of

897
00:44:32,660 --> 00:44:37,090
these charts with only seeing
the labels up here -- tell us

898
00:44:37,090 --> 00:44:40,430
the formula type, tell us the
steric number, tell us the

899
00:44:40,430 --> 00:44:43,190
geometry, you should be able to
draw the Lewis structure,

900
00:44:43,190 --> 00:44:45,200
you already know how to
do that, and also

901
00:44:45,200 --> 00:44:47,020
talk about the angles.

902
00:44:47,020 --> 00:44:50,900
So, as you finish up your
problem-set, you can, I think

903
00:44:50,900 --> 00:44:53,220
now, get through just
about all of it.

904
00:44:53,220 --> 00:44:55,720
You can do now this part
with the geometry.

905
00:44:55,720 --> 00:44:57,970
So, we'll see you on Wednesday
-- we're going to get out a

906
00:44:57,970 --> 00:44:59,940
little bit early today.