[Script Info]
Title: 
[Events]
Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
Dialogue: 0,0:00:07.27,0:00:09.50,Default,,0000,0000,0000,,What is the shape of a molecule?
Dialogue: 0,0:00:09.50,0:00:12.17,Default,,0000,0000,0000,,Well, a molecule is mostly empty space.
Dialogue: 0,0:00:12.17,0:00:14.00,Default,,0000,0000,0000,,Almost all of its mass is concentrated
Dialogue: 0,0:00:14.00,0:00:17.37,Default,,0000,0000,0000,,in the extremely dense nuclei of its atoms.
Dialogue: 0,0:00:17.37,0:00:18.31,Default,,0000,0000,0000,,And its electrons,
Dialogue: 0,0:00:18.31,0:00:19.79,Default,,0000,0000,0000,,which determine how the atoms
Dialogue: 0,0:00:19.79,0:00:20.78,Default,,0000,0000,0000,,are bonded to each other,
Dialogue: 0,0:00:20.78,0:00:23.31,Default,,0000,0000,0000,,are more like clouds of negative charge
Dialogue: 0,0:00:23.31,0:00:25.58,Default,,0000,0000,0000,,than individual, discrete particles.
Dialogue: 0,0:00:25.58,0:00:27.39,Default,,0000,0000,0000,,So, a molecule doesn't have a shape
Dialogue: 0,0:00:27.39,0:00:29.14,Default,,0000,0000,0000,,in the same way that, for example,
Dialogue: 0,0:00:29.14,0:00:31.01,Default,,0000,0000,0000,,a statue has a shape.
Dialogue: 0,0:00:31.01,0:00:32.08,Default,,0000,0000,0000,,But for every molecule,
Dialogue: 0,0:00:32.08,0:00:33.46,Default,,0000,0000,0000,,there's at least one way
Dialogue: 0,0:00:33.46,0:00:35.51,Default,,0000,0000,0000,,to arrange the nuclei and electrons
Dialogue: 0,0:00:35.51,0:00:37.71,Default,,0000,0000,0000,,so as to maximize the attraction
Dialogue: 0,0:00:37.71,0:00:38.85,Default,,0000,0000,0000,,of opposite charges
Dialogue: 0,0:00:38.85,0:00:40.39,Default,,0000,0000,0000,,and minimize the repulsion
Dialogue: 0,0:00:40.39,0:00:42.60,Default,,0000,0000,0000,,of like charges.
Dialogue: 0,0:00:42.60,0:00:44.35,Default,,0000,0000,0000,,Now, let's assume that the only electrons
Dialogue: 0,0:00:44.35,0:00:45.89,Default,,0000,0000,0000,,that matter to a molecule's shape
Dialogue: 0,0:00:45.89,0:00:48.81,Default,,0000,0000,0000,,are the outermost ones from each participating atom.
Dialogue: 0,0:00:49.45,0:00:50.50,Default,,0000,0000,0000,,And let's also assume
Dialogue: 0,0:00:50.50,0:00:52.89,Default,,0000,0000,0000,,that the electron clouds in between atoms,
Dialogue: 0,0:00:52.89,0:00:54.56,Default,,0000,0000,0000,,in other words, a molecule's bonds,
Dialogue: 0,0:00:54.56,0:00:57.47,Default,,0000,0000,0000,,are shaped kind of like sausages.
Dialogue: 0,0:00:57.47,0:01:00.29,Default,,0000,0000,0000,,Remember that nuclei are positively charged
Dialogue: 0,0:01:00.29,0:01:02.24,Default,,0000,0000,0000,,and electrons are negatively charged,
Dialogue: 0,0:01:02.24,0:01:03.73,Default,,0000,0000,0000,,and if all of a molecule's nuclei
Dialogue: 0,0:01:03.73,0:01:04.82,Default,,0000,0000,0000,,were bunched up together
Dialogue: 0,0:01:04.82,0:01:06.99,Default,,0000,0000,0000,,or all of its electrons were bunched up together,
Dialogue: 0,0:01:06.99,0:01:09.23,Default,,0000,0000,0000,,they would just repel each other and fly apart,
Dialogue: 0,0:01:09.23,0:01:11.00,Default,,0000,0000,0000,,and that doesn't help anyone.
Dialogue: 0,0:01:11.45,0:01:14.10,Default,,0000,0000,0000,,In 1776, Alessandro Volta,
Dialogue: 0,0:01:14.10,0:01:16.60,Default,,0000,0000,0000,,decades before he would eventually invent batteries,
Dialogue: 0,0:01:16.60,0:01:18.26,Default,,0000,0000,0000,,discovered methane.
Dialogue: 0,0:01:18.26,0:01:22.13,Default,,0000,0000,0000,,Now, the chemical formula of methane is CH4.
Dialogue: 0,0:01:22.13,0:01:23.13,Default,,0000,0000,0000,,And this formula tells us
Dialogue: 0,0:01:23.13,0:01:24.80,Default,,0000,0000,0000,,that every molecule of methane
Dialogue: 0,0:01:24.80,0:01:28.44,Default,,0000,0000,0000,,is made up of one carbon and four hydrogen atoms,
Dialogue: 0,0:01:28.44,0:01:31.14,Default,,0000,0000,0000,,but it doesn't tell us what's bonded to what
Dialogue: 0,0:01:31.14,0:01:34.53,Default,,0000,0000,0000,,or how they atoms are arranged in 3D space.
Dialogue: 0,0:01:34.53,0:01:36.21,Default,,0000,0000,0000,,From their electron configurations,
Dialogue: 0,0:01:36.21,0:01:37.71,Default,,0000,0000,0000,,we know that carbon can bond
Dialogue: 0,0:01:37.71,0:01:39.53,Default,,0000,0000,0000,,with up to four other atoms
Dialogue: 0,0:01:39.53,0:01:41.58,Default,,0000,0000,0000,,and that each hydrogen can only bond
Dialogue: 0,0:01:41.58,0:01:43.03,Default,,0000,0000,0000,,with one other atom.
Dialogue: 0,0:01:43.03,0:01:44.40,Default,,0000,0000,0000,,So, we can guess
Dialogue: 0,0:01:44.40,0:01:46.29,Default,,0000,0000,0000,,that the carbon should be the central atom
Dialogue: 0,0:01:46.29,0:01:48.91,Default,,0000,0000,0000,,bonded to all the hydrogens.
Dialogue: 0,0:01:48.91,0:01:50.04,Default,,0000,0000,0000,,Now, each bond represents
Dialogue: 0,0:01:50.04,0:01:51.66,Default,,0000,0000,0000,,the sharing of two electrons
Dialogue: 0,0:01:51.66,0:01:54.57,Default,,0000,0000,0000,,and we draw each shared pair of electrons as a line.
Dialogue: 0,0:01:54.57,0:01:56.80,Default,,0000,0000,0000,,So, now we have a flat representation
Dialogue: 0,0:01:56.80,0:01:58.26,Default,,0000,0000,0000,,of this molecule,
Dialogue: 0,0:01:58.26,0:02:00.83,Default,,0000,0000,0000,,but how would it look in three dimensions?
Dialogue: 0,0:02:00.83,0:02:01.81,Default,,0000,0000,0000,,We can reasonably say
Dialogue: 0,0:02:01.81,0:02:03.27,Default,,0000,0000,0000,,that because each of these bonds
Dialogue: 0,0:02:03.27,0:02:05.60,Default,,0000,0000,0000,,is a region of negative electric charge
Dialogue: 0,0:02:05.60,0:02:07.40,Default,,0000,0000,0000,,and like charges repel each other,
Dialogue: 0,0:02:07.40,0:02:09.57,Default,,0000,0000,0000,,the most favorable configuration of atoms
Dialogue: 0,0:02:09.57,0:02:12.33,Default,,0000,0000,0000,,would maximize the distance between bonds.
Dialogue: 0,0:02:12.33,0:02:13.74,Default,,0000,0000,0000,,And to get all the bonds
Dialogue: 0,0:02:13.74,0:02:16.07,Default,,0000,0000,0000,,as far away from each other as possible,
Dialogue: 0,0:02:16.07,0:02:18.51,Default,,0000,0000,0000,,the optimal shape is this.
Dialogue: 0,0:02:18.51,0:02:20.86,Default,,0000,0000,0000,,This is called a tetrahedron.
Dialogue: 0,0:02:20.86,0:02:22.90,Default,,0000,0000,0000,,Now, depending on the different atoms involved,
Dialogue: 0,0:02:22.90,0:02:25.32,Default,,0000,0000,0000,,you can actually get lots of different shapes.
Dialogue: 0,0:02:25.32,0:02:28.30,Default,,0000,0000,0000,,Ammonia, or NH3, is shaped like a pyramid.
Dialogue: 0,0:02:28.30,0:02:31.12,Default,,0000,0000,0000,,Carbon dioxide, or CO2, is a straight line.
Dialogue: 0,0:02:31.12,0:02:34.55,Default,,0000,0000,0000,,Water, H2O, is bent like your elbow would be bent.
Dialogue: 0,0:02:34.55,0:02:37.13,Default,,0000,0000,0000,,And chlorine trifluoride, or ClF3,
Dialogue: 0,0:02:37.13,0:02:39.22,Default,,0000,0000,0000,,is shaped like the letter T.
Dialogue: 0,0:02:39.22,0:02:40.91,Default,,0000,0000,0000,,Remember that what we've been doing here
Dialogue: 0,0:02:40.91,0:02:43.56,Default,,0000,0000,0000,,is expanding on our model of atoms and electrons
Dialogue: 0,0:02:43.56,0:02:45.84,Default,,0000,0000,0000,,to build up to 3D shapes.
Dialogue: 0,0:02:45.84,0:02:46.94,Default,,0000,0000,0000,,We'd have to do experiments
Dialogue: 0,0:02:46.94,0:02:48.14,Default,,0000,0000,0000,,to figure out if these molecules
Dialogue: 0,0:02:48.14,0:02:50.49,Default,,0000,0000,0000,,actually do have the shapes we predict.
Dialogue: 0,0:02:50.49,0:02:51.36,Default,,0000,0000,0000,,Spoiler alert:
Dialogue: 0,0:02:51.36,0:02:53.55,Default,,0000,0000,0000,,most of the do, but some of them don't.
Dialogue: 0,0:02:53.55,0:02:54.94,Default,,0000,0000,0000,,Now, shapes get more complicated
Dialogue: 0,0:02:54.94,0:02:56.94,Default,,0000,0000,0000,,as you increase the number of atoms.
Dialogue: 0,0:02:56.94,0:02:58.57,Default,,0000,0000,0000,,All the examples we just talked about
Dialogue: 0,0:02:58.57,0:03:01.07,Default,,0000,0000,0000,,had one obviously central atom,
Dialogue: 0,0:03:01.07,0:03:02.32,Default,,0000,0000,0000,,but most molecules,
Dialogue: 0,0:03:02.32,0:03:03.95,Default,,0000,0000,0000,,from relatively small pharmaceuticals
Dialogue: 0,0:03:03.95,0:03:05.37,Default,,0000,0000,0000,,all the way up to long polymers
Dialogue: 0,0:03:05.37,0:03:07.74,Default,,0000,0000,0000,,like DNA or proteins, don't.
Dialogue: 0,0:03:07.74,0:03:08.81,Default,,0000,0000,0000,,The key thing to remember
Dialogue: 0,0:03:08.81,0:03:10.88,Default,,0000,0000,0000,,is that bonded atoms will arrange themselves
Dialogue: 0,0:03:10.88,0:03:13.61,Default,,0000,0000,0000,,to maximize the attraction between opposite charges
Dialogue: 0,0:03:13.61,0:03:16.72,Default,,0000,0000,0000,,and minimize the repulsion between like charges.
Dialogue: 0,0:03:16.72,0:03:18.97,Default,,0000,0000,0000,,Some molecules even have two or more
Dialogue: 0,0:03:18.97,0:03:20.51,Default,,0000,0000,0000,,stable arrangements of atoms,
Dialogue: 0,0:03:20.51,0:03:22.47,Default,,0000,0000,0000,,and we can actually get really cool chemistry
Dialogue: 0,0:03:22.47,0:03:25.19,Default,,0000,0000,0000,,from the switches between those configurations,
Dialogue: 0,0:03:25.19,0:03:27.31,Default,,0000,0000,0000,,even when the composition of that molecule,
Dialogue: 0,0:03:27.31,0:03:29.89,Default,,0000,0000,0000,,that's to say the number and identity of its atoms,
Dialogue: 0,0:03:29.89,0:03:32.18,Default,,0000,0000,0000,,has not changed at all.