What is the shape of a molecule? - George Zaidan and Charles Morton

323,824 views ・ 2013-10-17

TED-Ed


Please double-click on the English subtitles below to play the video.

00:07
What is the shape of a molecule?
0
7270
2227
00:09
Well, a molecule is mostly empty space.
1
9497
2670
00:12
Almost all of its mass is concentrated
2
12167
1834
00:14
in the extremely dense nuclei of its atoms.
3
14001
3367
00:17
And its electrons,
4
17368
939
00:18
which determine how the atoms
5
18307
1483
00:19
are bonded to each other,
6
19790
994
00:20
are more like clouds of negative charge
7
20784
2523
00:23
than individual, discrete particles.
8
23307
2277
00:25
So, a molecule doesn't have a shape
9
25584
1810
00:27
in the same way that, for example,
10
27394
1745
00:29
a statue has a shape.
11
29139
1867
00:31
But for every molecule,
12
31006
1075
00:32
there's at least one way
13
32081
1375
00:33
to arrange the nuclei and electrons
14
33456
2055
00:35
so as to maximize the attraction
15
35511
2201
00:37
of opposite charges
16
37712
1138
00:38
and minimize the repulsion
17
38850
1536
00:40
of like charges.
18
40386
2213
00:42
Now, let's assume that the only electrons
19
42599
1748
00:44
that matter to a molecule's shape
20
44347
1544
00:45
are the outermost ones from each participating atom.
21
45891
2917
00:49
And let's also assume
22
49454
1050
00:50
that the electron clouds in between atoms,
23
50504
2389
00:52
in other words, a molecule's bonds,
24
52893
1668
00:54
are shaped kind of like sausages.
25
54561
2908
00:57
Remember that nuclei are positively charged
26
57469
2820
01:00
and electrons are negatively charged,
27
60289
1956
01:02
and if all of a molecule's nuclei
28
62245
1486
01:03
were bunched up together
29
63731
1090
01:04
or all of its electrons were bunched up together,
30
64821
2168
01:06
they would just repel each other and fly apart,
31
66989
2244
01:09
and that doesn't help anyone.
32
69233
1772
01:11
In 1776, Alessandro Volta,
33
71451
2652
01:14
decades before he would eventually invent batteries,
34
74103
2496
01:16
discovered methane.
35
76599
1664
01:18
Now, the chemical formula of methane is CH4.
36
78263
3870
01:22
And this formula tells us
37
82133
993
01:23
that every molecule of methane
38
83126
1674
01:24
is made up of one carbon and four hydrogen atoms,
39
84800
3642
01:28
but it doesn't tell us what's bonded to what
40
88442
2697
01:31
or how they atoms are arranged in 3D space.
41
91139
3393
01:34
From their electron configurations,
42
94532
1680
01:36
we know that carbon can bond
43
96212
1494
01:37
with up to four other atoms
44
97706
1825
01:39
and that each hydrogen can only bond
45
99531
2045
01:41
with one other atom.
46
101576
1458
01:43
So, we can guess
47
103034
1368
01:44
that the carbon should be the central atom
48
104402
1886
01:46
bonded to all the hydrogens.
49
106288
2618
01:48
Now, each bond represents
50
108906
1134
01:50
the sharing of two electrons
51
110040
1618
01:51
and we draw each shared pair of electrons as a line.
52
111658
2912
01:54
So, now we have a flat representation
53
114570
2231
01:56
of this molecule,
54
116801
1463
01:58
but how would it look in three dimensions?
55
118264
2562
02:00
We can reasonably say
56
120826
981
02:01
that because each of these bonds
57
121807
1462
02:03
is a region of negative electric charge
58
123269
2326
02:05
and like charges repel each other,
59
125595
1808
02:07
the most favorable configuration of atoms
60
127403
2166
02:09
would maximize the distance between bonds.
61
129569
2761
02:12
And to get all the bonds
62
132330
1413
02:13
as far away from each other as possible,
63
133743
2328
02:16
the optimal shape is this.
64
136071
2441
02:18
This is called a tetrahedron.
65
138512
2346
02:20
Now, depending on the different atoms involved,
66
140858
2043
02:22
you can actually get lots of different shapes.
67
142901
2422
02:25
Ammonia, or NH3, is shaped like a pyramid.
68
145323
2976
02:28
Carbon dioxide, or CO2, is a straight line.
69
148299
2823
02:31
Water, H2O, is bent like your elbow would be bent.
70
151122
3426
02:34
And chlorine trifluoride, or ClF3,
71
154548
2581
02:37
is shaped like the letter T.
72
157129
2086
02:39
Remember that what we've been doing here
73
159215
1694
02:40
is expanding on our model of atoms and electrons
74
160909
2652
02:43
to build up to 3D shapes.
75
163561
2282
02:45
We'd have to do experiments
76
165843
1094
02:46
to figure out if these molecules
77
166937
1201
02:48
actually do have the shapes we predict.
78
168138
2351
02:50
Spoiler alert:
79
170489
873
02:51
most of the do, but some of them don't.
80
171362
2192
02:53
Now, shapes get more complicated
81
173554
1384
02:54
as you increase the number of atoms.
82
174938
1999
02:56
All the examples we just talked about
83
176937
1637
02:58
had one obviously central atom,
84
178574
2497
03:01
but most molecules,
85
181071
1254
03:02
from relatively small pharmaceuticals
86
182325
1623
03:03
all the way up to long polymers
87
183948
1426
03:05
like DNA or proteins, don't.
88
185374
2369
03:07
The key thing to remember
89
187743
1065
03:08
is that bonded atoms will arrange themselves
90
188808
2071
03:10
to maximize the attraction between opposite charges
91
190879
2733
03:13
and minimize the repulsion between like charges.
92
193612
3105
03:16
Some molecules even have two or more
93
196717
2252
03:18
stable arrangements of atoms,
94
198969
1539
03:20
and we can actually get really cool chemistry
95
200508
1962
03:22
from the switches between those configurations,
96
202470
2720
03:25
even when the composition of that molecule,
97
205190
2116
03:27
that's to say the number and identity of its atoms,
98
207306
2588
03:29
has not changed at all.
99
209894
2291
About this website

This site will introduce you to YouTube videos that are useful for learning English. You will see English lessons taught by top-notch teachers from around the world. Double-click on the English subtitles displayed on each video page to play the video from there. The subtitles scroll in sync with the video playback. If you have any comments or requests, please contact us using this contact form.

https://forms.gle/WvT1wiN1qDtmnspy7