What is the shape of a molecule? - George Zaidan and Charles Morton
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0:07 - 0:09What is the shape of a molecule?
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0:09 - 0:12Well, a molecule is mostly empty space.
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0:12 - 0:14Almost all of its mass is concentrated
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0:14 - 0:17in the extremely dense nuclei of its atoms.
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0:17 - 0:18And its electrons,
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0:18 - 0:20which determine how the atoms
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0:20 - 0:21are bonded to each other,
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0:21 - 0:23are more like clouds of negative charge
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0:23 - 0:26than individual, discrete particles.
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0:26 - 0:27So, a molecule doesn't have a shape
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0:27 - 0:29in the same way that, for example,
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0:29 - 0:31a statue has a shape.
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0:31 - 0:32But for every molecule,
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0:32 - 0:33there's at least one way
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0:33 - 0:36to arrange the nuclei and electrons
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0:36 - 0:38so as to maximize the attraction
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0:38 - 0:39of opposite charges
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0:39 - 0:40and minimize the repulsion
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0:40 - 0:43of like charges.
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0:43 - 0:44Now, let's assume that the only electrons
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0:44 - 0:46that matter to a molecule's shape
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0:46 - 0:49are the outermost ones from each participating atom.
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0:49 - 0:51And let's also assume
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0:51 - 0:53that the electron clouds in between atoms,
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0:53 - 0:55in other words, a molecule's bonds,
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0:55 - 0:57are shaped kind of like sausages.
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0:57 - 1:00Remember that nuclei are positively charged
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1:00 - 1:02and electrons are negatively charged,
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1:02 - 1:04and if all of a molecule's nuclei
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1:04 - 1:05were bunched up together
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1:05 - 1:07or all of its electrons were bunched up together,
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1:07 - 1:09they would just repel each other and fly apart,
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1:09 - 1:11and that doesn't help anyone.
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1:11 - 1:14In 1776, Alessandro Volta,
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1:14 - 1:17decades before he would eventually invent batteries,
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1:17 - 1:18discovered methane.
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1:18 - 1:22Now, the chemical formula of methane is CH4.
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1:22 - 1:23And this formula tells us
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1:23 - 1:25that every molecule of methane
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1:25 - 1:28is made up of one carbon and four hydrogen atoms,
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1:28 - 1:31but it doesn't tell us what's bonded to what
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1:31 - 1:35or how they atoms are arranged in 3D space.
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1:35 - 1:36From their electron configurations,
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1:36 - 1:38we know that carbon can bond
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1:38 - 1:40with up to four other atoms
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1:40 - 1:42and that each hydrogen can only bond
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1:42 - 1:43with one other atom.
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1:43 - 1:44So, we can guess
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1:44 - 1:46that the carbon should be the central atom
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1:46 - 1:49bonded to all the hydrogens.
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1:49 - 1:50Now, each bond represents
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1:50 - 1:52the sharing of two electrons
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1:52 - 1:55and we draw each shared pair of electrons as a line.
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1:55 - 1:57So, now we have a flat representation
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1:57 - 1:58of this molecule,
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1:58 - 2:01but how would it look in three dimensions?
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2:01 - 2:02We can reasonably say
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2:02 - 2:03that because each of these bonds
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2:03 - 2:06is a region of negative electric charge
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2:06 - 2:07and like charges repel each other,
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2:07 - 2:10the most favorable configuration of atoms
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2:10 - 2:12would maximize the distance between bonds.
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2:12 - 2:14And to get all the bonds
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2:14 - 2:16as far away from each other as possible,
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2:16 - 2:19the optimal shape is this.
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2:19 - 2:21This is called a tetrahedron.
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2:21 - 2:23Now, depending on the different atoms involved,
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2:23 - 2:25you can actually get lots of different shapes.
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2:25 - 2:28Ammonia, or NH3, is shaped like a pyramid.
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2:28 - 2:31Carbon dioxide, or CO2, is a straight line.
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2:31 - 2:35Water, H2O, is bent like your elbow would be bent.
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2:35 - 2:37And chlorine trifluoride, or ClF3,
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2:37 - 2:39is shaped like the letter T.
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2:39 - 2:41Remember that what we've been doing here
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2:41 - 2:44is expanding on our model of atoms and electrons
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2:44 - 2:46to build up to 3D shapes.
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2:46 - 2:47We'd have to do experiments
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2:47 - 2:48to figure out if these molecules
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2:48 - 2:50actually do have the shapes we predict.
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2:50 - 2:51Spoiler alert:
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2:51 - 2:54most of the do, but some of them don't.
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2:54 - 2:55Now, shapes get more complicated
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2:55 - 2:57as you increase the number of atoms.
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2:57 - 2:59All the examples we just talked about
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2:59 - 3:01had one obviously central atom,
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3:01 - 3:02but most molecules,
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3:02 - 3:04from relatively small pharmaceuticals
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3:04 - 3:05all the way up to long polymers
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3:05 - 3:08like DNA or proteins, don't.
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3:08 - 3:09The key thing to remember
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3:09 - 3:11is that bonded atoms will arrange themselves
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3:11 - 3:14to maximize the attraction between opposite charges
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3:14 - 3:17and minimize the repulsion between like charges.
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3:17 - 3:19Some molecules even have two or more
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3:19 - 3:21stable arrangements of atoms,
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3:21 - 3:22and we can actually get really cool chemistry
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3:22 - 3:25from the switches between those configurations,
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3:25 - 3:27even when the composition of that molecule,
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3:27 - 3:30that's to say the number and identity of its atoms,
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3:30 - 3:32has not changed at all.
- Title:
- What is the shape of a molecule? - George Zaidan and Charles Morton
- Description:
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View full lesson: http://ed.ted.com/lessons/what-is-the-shape-of-a-molecule-george-zaidan-and-charles-morton
A molecule is nearly all empty space, apart from the extremely dense nuclei of its atoms and the clouds of electrons that bond them together. When that molecule forms, it arranges itself to maximize attraction of opposite charges and minimize repulsion of unlike. George Zaidan and Charles Morton shape our image of molecules.
Lesson by George Zaidan and Charles Morton, animation by Bevan Lynch.
- Video Language:
- English
- Team:
- closed TED
- Project:
- TED-Ed
- Duration:
- 03:48
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Andrea McDonough edited English subtitles for What is the shape of a molecule? - George Zaidan and Charles Morton |