[Script Info]
Title: 
[Events]
Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
Dialogue: 0,0:00:00.00,0:00:02.07,Default,,0000,0000,0000,,- [Teacher] To figure out\Nhow we use semiconductors
Dialogue: 0,0:00:02.07,0:00:04.53,Default,,0000,0000,0000,,to build all these\Nawesome computing devices,
Dialogue: 0,0:00:04.53,0:00:06.21,Default,,0000,0000,0000,,we're going to start from scratch,
Dialogue: 0,0:00:06.21,0:00:08.62,Default,,0000,0000,0000,,all the way down to even understanding
Dialogue: 0,0:00:08.62,0:00:11.52,Default,,0000,0000,0000,,why semiconductors are semiconductors.
Dialogue: 0,0:00:11.52,0:00:13.07,Default,,0000,0000,0000,,I mean, why is it that certain materials
Dialogue: 0,0:00:13.07,0:00:13.99,Default,,0000,0000,0000,,behave like conductors,
Dialogue: 0,0:00:13.99,0:00:16.53,Default,,0000,0000,0000,,which are very good at passing electricity
Dialogue: 0,0:00:16.53,0:00:18.87,Default,,0000,0000,0000,,through them while others are not?
Dialogue: 0,0:00:18.87,0:00:23.57,Default,,0000,0000,0000,,To understand this, we need\Nto look at the atomic level.
Dialogue: 0,0:00:23.57,0:00:26.49,Default,,0000,0000,0000,,Now we might have some\Nintuition about these atoms,
Dialogue: 0,0:00:26.49,0:00:27.32,Default,,0000,0000,0000,,but guess what?
Dialogue: 0,0:00:27.32,0:00:28.42,Default,,0000,0000,0000,,Turns out that our knowledge
Dialogue: 0,0:00:28.42,0:00:30.90,Default,,0000,0000,0000,,of the atomic structure is not enough.
Dialogue: 0,0:00:30.90,0:00:32.85,Default,,0000,0000,0000,,And so in this video, we're\Njust gonna recapitulate
Dialogue: 0,0:00:32.85,0:00:34.65,Default,,0000,0000,0000,,all the stuff that we might already know
Dialogue: 0,0:00:34.65,0:00:35.83,Default,,0000,0000,0000,,from the previous videos.
Dialogue: 0,0:00:35.83,0:00:38.62,Default,,0000,0000,0000,,And we'll see why the current knowledge
Dialogue: 0,0:00:38.62,0:00:39.83,Default,,0000,0000,0000,,or the current theory of the atoms
Dialogue: 0,0:00:39.83,0:00:44.00,Default,,0000,0000,0000,,is not sufficient to talk\Nabout solids in general,
Dialogue: 0,0:00:44.00,0:00:45.45,Default,,0000,0000,0000,,which we'll be interested in.
Dialogue: 0,0:00:46.62,0:00:48.89,Default,,0000,0000,0000,,For starters, you may\Nalready have some intuition.
Dialogue: 0,0:00:48.89,0:00:53.16,Default,,0000,0000,0000,,For example, you may know that\Nall matter is made of atoms.
Dialogue: 0,0:00:53.16,0:00:56.90,Default,,0000,0000,0000,,And if you were to pick any\None of them and zoom in,
Dialogue: 0,0:00:56.90,0:00:59.10,Default,,0000,0000,0000,,then you might know that\Nthe atoms themselves
Dialogue: 0,0:00:59.10,0:01:01.11,Default,,0000,0000,0000,,are made of even smaller things.
Dialogue: 0,0:01:01.11,0:01:03.37,Default,,0000,0000,0000,,At the center, we have this\Nthing called as the nucleus,
Dialogue: 0,0:01:03.37,0:01:05.00,Default,,0000,0000,0000,,which have a positive charge,
Dialogue: 0,0:01:05.00,0:01:07.66,Default,,0000,0000,0000,,and the electrons which\Nare negatively charged
Dialogue: 0,0:01:07.66,0:01:08.99,Default,,0000,0000,0000,,are attracted by the nucleus
Dialogue: 0,0:01:08.99,0:01:12.21,Default,,0000,0000,0000,,and end up going around the\Nnucleus in different orbits
Dialogue: 0,0:01:12.21,0:01:13.62,Default,,0000,0000,0000,,just like the solar system
Dialogue: 0,0:01:13.62,0:01:15.46,Default,,0000,0000,0000,,and how the planets go around the sun.
Dialogue: 0,0:01:15.46,0:01:17.29,Default,,0000,0000,0000,,Now this is not a very accurate model,
Dialogue: 0,0:01:17.29,0:01:18.85,Default,,0000,0000,0000,,we'll get back to that.
Dialogue: 0,0:01:18.85,0:01:20.65,Default,,0000,0000,0000,,But as of now, let's use this model.
Dialogue: 0,0:01:20.65,0:01:23.49,Default,,0000,0000,0000,,But the important thing is\Nthere are some electrons
Dialogue: 0,0:01:23.49,0:01:26.69,Default,,0000,0000,0000,,like these, which are\Ntightly bound to the nucleus.
Dialogue: 0,0:01:26.69,0:01:28.60,Default,,0000,0000,0000,,We call them as bound electrons.
Dialogue: 0,0:01:29.62,0:01:31.75,Default,,0000,0000,0000,,Bound electrons, and\Nthese are not responsible
Dialogue: 0,0:01:31.75,0:01:33.07,Default,,0000,0000,0000,,for conduction.
Dialogue: 0,0:01:33.07,0:01:35.30,Default,,0000,0000,0000,,Whereas there are other electrons
Dialogue: 0,0:01:35.30,0:01:37.74,Default,,0000,0000,0000,,which are not strongly\Nattracted by the nucleus
Dialogue: 0,0:01:37.74,0:01:41.21,Default,,0000,0000,0000,,and they are free, as\Nin, they're free to move
Dialogue: 0,0:01:41.21,0:01:43.28,Default,,0000,0000,0000,,from one atom to another.
Dialogue: 0,0:01:43.28,0:01:45.80,Default,,0000,0000,0000,,And it's these electrons which we call
Dialogue: 0,0:01:45.80,0:01:47.58,Default,,0000,0000,0000,,as conduction electrons or free electrons,
Dialogue: 0,0:01:47.58,0:01:50.59,Default,,0000,0000,0000,,which are really\Nresponsible for conduction.
Dialogue: 0,0:01:50.59,0:01:51.71,Default,,0000,0000,0000,,And in some materials,
Dialogue: 0,0:01:51.71,0:01:54.64,Default,,0000,0000,0000,,it's very easy to get\Nthese free electrons.
Dialogue: 0,0:01:54.64,0:01:56.84,Default,,0000,0000,0000,,And so they end up having a lot of them,
Dialogue: 0,0:01:56.84,0:01:59.44,Default,,0000,0000,0000,,and we call these materials\Nas good conductors
Dialogue: 0,0:01:59.44,0:02:00.61,Default,,0000,0000,0000,,or conductors.
Dialogue: 0,0:02:00.61,0:02:02.14,Default,,0000,0000,0000,,On the other hand, some materials,
Dialogue: 0,0:02:02.14,0:02:04.69,Default,,0000,0000,0000,,well, it's extremely difficult\Nto get these free electrons.
Dialogue: 0,0:02:04.69,0:02:07.90,Default,,0000,0000,0000,,And as a result, you have\Nextremely negligible amount.
Dialogue: 0,0:02:07.90,0:02:11.34,Default,,0000,0000,0000,,And as a result, they are\Nbad conductors or insulators.
Dialogue: 0,0:02:11.34,0:02:13.15,Default,,0000,0000,0000,,And of course we have\Nthe intermediate ones
Dialogue: 0,0:02:13.15,0:02:15.28,Default,,0000,0000,0000,,which we end up calling semiconductors.
Dialogue: 0,0:02:15.28,0:02:16.60,Default,,0000,0000,0000,,So I think the most important question
Dialogue: 0,0:02:16.60,0:02:18.54,Default,,0000,0000,0000,,that we have to ask ourselves over here,
Dialogue: 0,0:02:18.54,0:02:21.06,Default,,0000,0000,0000,,is how does an electron become free?
Dialogue: 0,0:02:21.06,0:02:23.33,Default,,0000,0000,0000,,I mean, what makes it free\Nand what does that depend on?
Dialogue: 0,0:02:23.33,0:02:25.12,Default,,0000,0000,0000,,That's the thing that\Nwe need to figure out.
Dialogue: 0,0:02:25.12,0:02:27.80,Default,,0000,0000,0000,,And we have to look at,\Nlook at this whole thing
Dialogue: 0,0:02:27.80,0:02:31.61,Default,,0000,0000,0000,,for a solid, because our\Nsemiconductors are solids.
Dialogue: 0,0:02:31.61,0:02:33.65,Default,,0000,0000,0000,,So we need to find out,\Nor we need to figure out
Dialogue: 0,0:02:33.65,0:02:37.08,Default,,0000,0000,0000,,what makes an electron free in solids.
Dialogue: 0,0:02:37.08,0:02:39.85,Default,,0000,0000,0000,,And to do that, we need to get past this
Dialogue: 0,0:02:39.85,0:02:43.11,Default,,0000,0000,0000,,solar system model of the\Natom, as I mentioned before,
Dialogue: 0,0:02:43.11,0:02:44.30,Default,,0000,0000,0000,,it's not very accurate.
Dialogue: 0,0:02:44.30,0:02:47.28,Default,,0000,0000,0000,,And we need to look at\Na more accurate model
Dialogue: 0,0:02:47.28,0:02:49.51,Default,,0000,0000,0000,,of the atomic structure.
Dialogue: 0,0:02:49.51,0:02:50.97,Default,,0000,0000,0000,,So let's do that.
Dialogue: 0,0:02:50.97,0:02:53.68,Default,,0000,0000,0000,,Now, you may have already\Nlearned about this in chemistry.
Dialogue: 0,0:02:53.68,0:02:55.38,Default,,0000,0000,0000,,It turns out that instead of thinking
Dialogue: 0,0:02:55.38,0:02:58.44,Default,,0000,0000,0000,,of where the electrons are and what orbits
Dialogue: 0,0:02:58.44,0:02:59.85,Default,,0000,0000,0000,,or what path they take,
Dialogue: 0,0:02:59.85,0:03:03.34,Default,,0000,0000,0000,,it's much better to think about\Nthem in terms of energies.
Dialogue: 0,0:03:03.34,0:03:05.45,Default,,0000,0000,0000,,It's better think about\Nwhat are the energies
Dialogue: 0,0:03:05.45,0:03:07.00,Default,,0000,0000,0000,,that the electrons can take up.
Dialogue: 0,0:03:07.00,0:03:09.28,Default,,0000,0000,0000,,And you may have already\Nstudied in chemistry
Dialogue: 0,0:03:09.28,0:03:11.20,Default,,0000,0000,0000,,that the inside of any atoms,
Dialogue: 0,0:03:11.20,0:03:14.94,Default,,0000,0000,0000,,so if I draw over here energies,
Dialogue: 0,0:03:14.94,0:03:18.14,Default,,0000,0000,0000,,inside any atom, electrons\Ncan have only some
Dialogue: 0,0:03:18.14,0:03:20.10,Default,,0000,0000,0000,,specific energy values,
Dialogue: 0,0:03:20.10,0:03:22.88,Default,,0000,0000,0000,,only some specific energy values.
Dialogue: 0,0:03:22.88,0:03:24.72,Default,,0000,0000,0000,,And so maybe the lowest energy
Dialogue: 0,0:03:24.72,0:03:26.97,Default,,0000,0000,0000,,that electron can have\Nmaybe somewhere over here.
Dialogue: 0,0:03:26.97,0:03:29.31,Default,,0000,0000,0000,,We're not gonna write down\Nthe numbers over here.
Dialogue: 0,0:03:29.31,0:03:30.96,Default,,0000,0000,0000,,We're not gonna look at\Nit very quantitatively,
Dialogue: 0,0:03:30.96,0:03:32.01,Default,,0000,0000,0000,,don't worry about it.
Dialogue: 0,0:03:32.01,0:03:33.45,Default,,0000,0000,0000,,So maybe this is the lowest energy
Dialogue: 0,0:03:33.45,0:03:35.00,Default,,0000,0000,0000,,that an electron can possess.
Dialogue: 0,0:03:35.00,0:03:36.73,Default,,0000,0000,0000,,The next higher energy\Nan electron can possess
Dialogue: 0,0:03:36.73,0:03:38.45,Default,,0000,0000,0000,,might be somewhere over here,
Dialogue: 0,0:03:38.45,0:03:41.24,Default,,0000,0000,0000,,and maybe next higher might\Nbe somewhere over here,
Dialogue: 0,0:03:41.24,0:03:42.29,Default,,0000,0000,0000,,and so on and so forth.
Dialogue: 0,0:03:42.29,0:03:44.55,Default,,0000,0000,0000,,And we give names to these energy levels.
Dialogue: 0,0:03:44.55,0:03:48.57,Default,,0000,0000,0000,,We call the lowest one\Nas the 1S energy level.
Dialogue: 0,0:03:48.57,0:03:50.12,Default,,0000,0000,0000,,The next higher one becomes 2S,
Dialogue: 0,0:03:51.17,0:03:54.23,Default,,0000,0000,0000,,the one that comes above that would be 2P.
Dialogue: 0,0:03:54.23,0:03:58.98,Default,,0000,0000,0000,,Then we have 3S and 3P\Nand so on and so forth.
Dialogue: 0,0:03:58.98,0:04:01.78,Default,,0000,0000,0000,,And again, if this looks very new to you
Dialogue: 0,0:04:01.78,0:04:03.80,Default,,0000,0000,0000,,and you have no idea what S and P are,
Dialogue: 0,0:04:03.80,0:04:06.65,Default,,0000,0000,0000,,it would be a great idea\Nto pause this over here,
Dialogue: 0,0:04:06.65,0:04:09.36,Default,,0000,0000,0000,,go back and watch the\Nelectron configuration videos
Dialogue: 0,0:04:09.36,0:04:13.18,Default,,0000,0000,0000,,on chemistry, and then\Ncome back over here.
Dialogue: 0,0:04:13.18,0:04:15.76,Default,,0000,0000,0000,,But anyways, it turns out\Nelectrons cannot take up
Dialogue: 0,0:04:15.76,0:04:17.70,Default,,0000,0000,0000,,these energy levels randomly.
Dialogue: 0,0:04:17.70,0:04:20.34,Default,,0000,0000,0000,,There's a particular rule\Nusing which electrons
Dialogue: 0,0:04:20.34,0:04:23.31,Default,,0000,0000,0000,,sort of fill up these energy levels.
Dialogue: 0,0:04:23.31,0:04:25.54,Default,,0000,0000,0000,,And that rule, again, you\Nmay have studied about them.
Dialogue: 0,0:04:25.54,0:04:28.98,Default,,0000,0000,0000,,We call that as the Pauli's\Nexclusion principle.
Dialogue: 0,0:04:28.98,0:04:31.68,Default,,0000,0000,0000,,Pauli's exclusion,
Dialogue: 0,0:04:33.38,0:04:35.93,Default,,0000,0000,0000,,exclusion principle, or rule.
Dialogue: 0,0:04:35.93,0:04:39.15,Default,,0000,0000,0000,,And it simply says that no two electrons,
Dialogue: 0,0:04:39.15,0:04:41.21,Default,,0000,0000,0000,,no two electrons
Dialogue: 0,0:04:42.19,0:04:44.87,Default,,0000,0000,0000,,can have identical,
Dialogue: 0,0:04:44.87,0:04:48.95,Default,,0000,0000,0000,,can have identical energies.
Dialogue: 0,0:04:48.95,0:04:51.67,Default,,0000,0000,0000,,Now, again, this is not the\Naccurate statement of Pauli,
Dialogue: 0,0:04:51.67,0:04:54.32,Default,,0000,0000,0000,,but this will help us,\Nthis is enough for us.
Dialogue: 0,0:04:54.32,0:04:56.23,Default,,0000,0000,0000,,So let's take a concrete example.
Dialogue: 0,0:04:56.23,0:04:59.59,Default,,0000,0000,0000,,Suppose we take, say, a sodium atom,
Dialogue: 0,0:04:59.59,0:05:03.60,Default,,0000,0000,0000,,then it has, it has 11\Nelectrons inside it.
Dialogue: 0,0:05:03.60,0:05:05.39,Default,,0000,0000,0000,,There are 11 electrons.
Dialogue: 0,0:05:07.04,0:05:08.85,Default,,0000,0000,0000,,And now these 11 electrons
Dialogue: 0,0:05:08.85,0:05:11.48,Default,,0000,0000,0000,,can only have these\Nspecific energy levels.
Dialogue: 0,0:05:11.48,0:05:13.56,Default,,0000,0000,0000,,And the way these electrons
Dialogue: 0,0:05:13.56,0:05:15.20,Default,,0000,0000,0000,,are going to fill up the energy levels
Dialogue: 0,0:05:15.20,0:05:17.70,Default,,0000,0000,0000,,will be using the exclusion principle.
Dialogue: 0,0:05:17.70,0:05:21.25,Default,,0000,0000,0000,,So the first electron, well, remember,
Dialogue: 0,0:05:21.25,0:05:23.58,Default,,0000,0000,0000,,electrons always want to take\Nthe lowest energy possible.
Dialogue: 0,0:05:23.58,0:05:26.96,Default,,0000,0000,0000,,So the first electron would\Ngo over here, over here,
Dialogue: 0,0:05:26.96,0:05:28.61,Default,,0000,0000,0000,,and then you might think,\Nwell, the next electron
Dialogue: 0,0:05:28.61,0:05:31.05,Default,,0000,0000,0000,,can't go over here because\Nthat's what Pauli's telling us.
Dialogue: 0,0:05:31.05,0:05:32.41,Default,,0000,0000,0000,,No arguing with Pauli.
Dialogue: 0,0:05:32.41,0:05:34.11,Default,,0000,0000,0000,,Second electron, if it comes over here,
Dialogue: 0,0:05:34.11,0:05:37.26,Default,,0000,0000,0000,,it might have identical\Nenergy, but not really,
Dialogue: 0,0:05:37.26,0:05:40.70,Default,,0000,0000,0000,,because it turns out\Nthat electrons can have
Dialogue: 0,0:05:40.70,0:05:42.38,Default,,0000,0000,0000,,up spin and down spins.
Dialogue: 0,0:05:42.38,0:05:45.36,Default,,0000,0000,0000,,So if the first electron\Ngoes into the 1S tier,
Dialogue: 0,0:05:45.36,0:05:47.92,Default,,0000,0000,0000,,and suppose it takes up the up spin,
Dialogue: 0,0:05:47.92,0:05:50.37,Default,,0000,0000,0000,,then another electron can actually take up
Dialogue: 0,0:05:50.37,0:05:53.59,Default,,0000,0000,0000,,the same energy level and now be down spin
Dialogue: 0,0:05:53.59,0:05:54.95,Default,,0000,0000,0000,,because turns out these two spins
Dialogue: 0,0:05:54.95,0:05:57.10,Default,,0000,0000,0000,,have slightly different energy.
Dialogue: 0,0:05:57.10,0:05:59.24,Default,,0000,0000,0000,,So these two electrons\Nare strictly speaking,
Dialogue: 0,0:05:59.24,0:06:02.14,Default,,0000,0000,0000,,still being Pauli, because\Nthey're not exactly identical
Dialogue: 0,0:06:02.14,0:06:03.74,Default,,0000,0000,0000,,because of their spins.
Dialogue: 0,0:06:03.74,0:06:05.64,Default,,0000,0000,0000,,But the next electron, the third electron,
Dialogue: 0,0:06:05.64,0:06:08.65,Default,,0000,0000,0000,,well, it cannot take up the\N1S energy level anymore,
Dialogue: 0,0:06:08.65,0:06:10.64,Default,,0000,0000,0000,,because if it does and then up spin,
Dialogue: 0,0:06:10.64,0:06:11.88,Default,,0000,0000,0000,,then it'll be identical to this one.
Dialogue: 0,0:06:11.88,0:06:13.12,Default,,0000,0000,0000,,If it does with a down spin,
Dialogue: 0,0:06:13.12,0:06:15.10,Default,,0000,0000,0000,,then it'll be identical to this one.
Dialogue: 0,0:06:15.10,0:06:16.68,Default,,0000,0000,0000,,So it can't take the that up anywhere.
Dialogue: 0,0:06:16.68,0:06:18.15,Default,,0000,0000,0000,,So it has to take up now
Dialogue: 0,0:06:18.15,0:06:20.85,Default,,0000,0000,0000,,the next higher energy level\Navailable that's over here.
Dialogue: 0,0:06:20.85,0:06:22.73,Default,,0000,0000,0000,,It can take up anywhere\Nin between as well.
Dialogue: 0,0:06:22.73,0:06:24.96,Default,,0000,0000,0000,,The energy levels in\Nbetween are inaccessible
Dialogue: 0,0:06:24.96,0:06:25.79,Default,,0000,0000,0000,,to these electrons.
Dialogue: 0,0:06:25.79,0:06:28.28,Default,,0000,0000,0000,,So the next energy it\Nwill take up would be 2S,
Dialogue: 0,0:06:28.28,0:06:30.30,Default,,0000,0000,0000,,again, it might take up with an up spin.
Dialogue: 0,0:06:30.30,0:06:33.58,Default,,0000,0000,0000,,The fourth electron might\Ngo over with a down spin.
Dialogue: 0,0:06:33.58,0:06:36.91,Default,,0000,0000,0000,,The next electron will\Ntake up over here, up spin,
Dialogue: 0,0:06:36.91,0:06:38.76,Default,,0000,0000,0000,,and the next one will be down spin.
Dialogue: 0,0:06:39.70,0:06:40.85,Default,,0000,0000,0000,,Now here's the thing.
Dialogue: 0,0:06:40.85,0:06:44.06,Default,,0000,0000,0000,,It turns out that in P, in P energy level,
Dialogue: 0,0:06:44.06,0:06:46.63,Default,,0000,0000,0000,,there are three ways in which electrons
Dialogue: 0,0:06:46.63,0:06:48.42,Default,,0000,0000,0000,,can occupy that energy level.
Dialogue: 0,0:06:48.42,0:06:50.83,Default,,0000,0000,0000,,We call them as orbitals, right?
Dialogue: 0,0:06:50.83,0:06:52.96,Default,,0000,0000,0000,,It turns out that in the S energy levels,
Dialogue: 0,0:06:52.96,0:06:53.92,Default,,0000,0000,0000,,there's only one way.
Dialogue: 0,0:06:53.92,0:06:55.71,Default,,0000,0000,0000,,So there's only one orbital,
Dialogue: 0,0:06:55.71,0:06:57.90,Default,,0000,0000,0000,,but in P there are three orbitals.
Dialogue: 0,0:06:57.90,0:07:01.76,Default,,0000,0000,0000,,So another electron can\Ntake up the 2P energy level
Dialogue: 0,0:07:01.76,0:07:04.45,Default,,0000,0000,0000,,by being in a different orbital.
Dialogue: 0,0:07:04.45,0:07:05.91,Default,,0000,0000,0000,,So this electron and this electron
Dialogue: 0,0:07:05.91,0:07:07.66,Default,,0000,0000,0000,,will be in different orbitals,
Dialogue: 0,0:07:07.66,0:07:09.45,Default,,0000,0000,0000,,or different configuration, we could say,
Dialogue: 0,0:07:09.45,0:07:10.97,Default,,0000,0000,0000,,don't have to worry about it too much.
Dialogue: 0,0:07:10.97,0:07:12.93,Default,,0000,0000,0000,,And so they'll still not be identical.
Dialogue: 0,0:07:12.93,0:07:15.32,Default,,0000,0000,0000,,And so another electron can\Ntake up that same orbital
Dialogue: 0,0:07:15.32,0:07:16.75,Default,,0000,0000,0000,,with a down spin.
Dialogue: 0,0:07:16.75,0:07:21.17,Default,,0000,0000,0000,,Another electron, the third\Norbital of P with an up spin,
Dialogue: 0,0:07:21.17,0:07:22.88,Default,,0000,0000,0000,,and then down spin.
Dialogue: 0,0:07:22.88,0:07:25.04,Default,,0000,0000,0000,,And now the 2P is completely filled.
Dialogue: 0,0:07:25.04,0:07:28.17,Default,,0000,0000,0000,,There are no more orbitals available.
Dialogue: 0,0:07:28.17,0:07:29.19,Default,,0000,0000,0000,,And so the last electron,
Dialogue: 0,0:07:29.19,0:07:31.12,Default,,0000,0000,0000,,we're down to one, two,\Nthree, four, five, six, seven,
Dialogue: 0,0:07:31.12,0:07:35.09,Default,,0000,0000,0000,,eight, nine, 10, the last\Nelectron will be over here
Dialogue: 0,0:07:35.09,0:07:37.81,Default,,0000,0000,0000,,in the 3S up spin.
Dialogue: 0,0:07:37.81,0:07:41.54,Default,,0000,0000,0000,,But this is for a single atom of sodium.
Dialogue: 0,0:07:41.54,0:07:44.97,Default,,0000,0000,0000,,What if we have say, two atoms of sodium,
Dialogue: 0,0:07:44.97,0:07:47.41,Default,,0000,0000,0000,,very close to each\Nother, what happens then?
Dialogue: 0,0:07:49.09,0:07:49.92,Default,,0000,0000,0000,,Somewhat like this,
Dialogue: 0,0:07:49.92,0:07:52.06,Default,,0000,0000,0000,,what if they form some kind of a molecule?
Dialogue: 0,0:07:52.06,0:07:54.99,Default,,0000,0000,0000,,How would the electrons of this molecule
Dialogue: 0,0:07:54.99,0:07:56.31,Default,,0000,0000,0000,,fill up the energy levels?
Dialogue: 0,0:07:56.31,0:07:59.60,Default,,0000,0000,0000,,Can we say that now each atom\Nwill have something like this.
Dialogue: 0,0:07:59.60,0:08:03.61,Default,,0000,0000,0000,,Each atom will have electrons\Nfilled up accordingly.
Dialogue: 0,0:08:03.61,0:08:06.16,Default,,0000,0000,0000,,Well, that won't work,\Nthat can't be possible.
Dialogue: 0,0:08:06.16,0:08:07.29,Default,,0000,0000,0000,,And the way we can think about it,
Dialogue: 0,0:08:07.29,0:08:09.37,Default,,0000,0000,0000,,is we can say that, if you do it this way,
Dialogue: 0,0:08:09.37,0:08:11.36,Default,,0000,0000,0000,,Pauli's rule will be violated.
Dialogue: 0,0:08:11.36,0:08:13.85,Default,,0000,0000,0000,,Remember, Pauli says no two electrons,
Dialogue: 0,0:08:13.85,0:08:15.49,Default,,0000,0000,0000,,and when we say no two electrons,
Dialogue: 0,0:08:15.49,0:08:17.74,Default,,0000,0000,0000,,it can be no two electrons inside an atom,
Dialogue: 0,0:08:17.74,0:08:20.37,Default,,0000,0000,0000,,or no two electrons inside a molecule,
Dialogue: 0,0:08:20.37,0:08:23.96,Default,,0000,0000,0000,,or maybe no two electrons\Ninside an entire solid.
Dialogue: 0,0:08:23.96,0:08:26.49,Default,,0000,0000,0000,,No two electrons can\Nhave identical energies.
Dialogue: 0,0:08:26.49,0:08:30.04,Default,,0000,0000,0000,,So if the two atoms have\Nthese electron configurations
Dialogue: 0,0:08:30.04,0:08:32.39,Default,,0000,0000,0000,,then I hope you can see that this electron
Dialogue: 0,0:08:32.39,0:08:35.43,Default,,0000,0000,0000,,and this electron will,\Nthey will be identical.
Dialogue: 0,0:08:35.43,0:08:38.72,Default,,0000,0000,0000,,This one, and this one will\Nbe absolutely identical.
Dialogue: 0,0:08:38.72,0:08:40.96,Default,,0000,0000,0000,,And so all of them will\Nhave identical pairs
Dialogue: 0,0:08:40.96,0:08:45.36,Default,,0000,0000,0000,,and Pauli will be very, very\Nsad, so that can't be possible.
Dialogue: 0,0:08:45.36,0:08:48.08,Default,,0000,0000,0000,,And if we have an entire solid,
Dialogue: 0,0:08:48.08,0:08:51.71,Default,,0000,0000,0000,,which is made of sodium, where\Nwe have like 10 to the 23
Dialogue: 0,0:08:51.71,0:08:53.87,Default,,0000,0000,0000,,atoms packed very close to each other,
Dialogue: 0,0:08:53.87,0:08:57.52,Default,,0000,0000,0000,,and if we used this model for each atom,
Dialogue: 0,0:08:57.52,0:09:00.84,Default,,0000,0000,0000,,then there would be about 10\Nto the 23 identical copies
Dialogue: 0,0:09:00.84,0:09:03.34,Default,,0000,0000,0000,,of electrons in each level.
Dialogue: 0,0:09:03.34,0:09:08.02,Default,,0000,0000,0000,,And that would make Pauli\Nextremely sad, extremely sad.
Dialogue: 0,0:09:08.02,0:09:11.37,Default,,0000,0000,0000,,So the key takeaway is that this structure
Dialogue: 0,0:09:11.37,0:09:15.40,Default,,0000,0000,0000,,that we have learned for a\Nsingle atom cannot be extended
Dialogue: 0,0:09:15.40,0:09:16.83,Default,,0000,0000,0000,,when we go all the way to the solids.
Dialogue: 0,0:09:16.83,0:09:19.70,Default,,0000,0000,0000,,We require a new theory to\Nunderstand what's going on
Dialogue: 0,0:09:19.70,0:09:22.23,Default,,0000,0000,0000,,and how electrons are arranged\Nor how to think about them
Dialogue: 0,0:09:22.23,0:09:24.04,Default,,0000,0000,0000,,when it comes to solids.
Dialogue: 0,0:09:24.04,0:09:26.58,Default,,0000,0000,0000,,And we'll explore them\Nin the future videos.