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In the video where we introduced the atom, I went

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off a bit about how at the center of an atom we have the

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nucleus, and it's actually a very small fraction of the

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total volume of the atom.

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And the electron, even though we call it a particle, it can

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really be best described as kind of a

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smear around this nucleus.

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That although it's a particle, because of the Heisenberg

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uncertainty principle, we can never tell exactly at a given

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moment where the particle is and what its momentum is.

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So to describe it as a particle is a little bit, I

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don't know, at best, it's a little bit strange.

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And we said that the way that they describe it, they don't

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say that this particle is in an orbit, like the planets

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around the Sun in orbit would be like that.

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That would be like the orbit of Halley's

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Comet around the Sun.

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Instead, it can be described as a probability function

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around the nucleus.

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So if the nucleus is there, we have one orbital, actually the

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1s orbital, and we'll talk about that in this video.

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It'll be a sphere around the nucleus.

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And actually the sphere has no strict boundary.

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Whenever you see someone draw it, they're just saying, where

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is 90% of the time the electron going to be.

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And then they'll cut off a boundary.

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And they'll say, OK, it's going to be within this sphere

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and it actually gets denser as you get into the

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center of the sphere.

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So if this was a cross-section, it would be

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really dense in the center, and it gets less dense, less

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dense as you go outside.

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Which just means that there's a much higher probability of

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finding the electron in the 1s orbital near the center than

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near the outside.

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Although this boundary point out here is just artificial.

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You can find the electron pretty much anywhere.

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It just has a much lower probability out

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there than in here.

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But I'll touch on that in more detail in the

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rest of this video.

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But I wanted to go back to the Bohr model.

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And the Bohr model is the kind of-- let me write that down.

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Bohr model.

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And sometimes it's nice to know it's

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named after Niels Bohr.

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And don't think that this guy was some slouch.

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He was at the cutting edge and this wasn't

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even that long ago.

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This was roughly about 100 years ago.

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So already we're talking about things that you can probably

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dig up research papers in your library not too long ago where

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people are debating some of these issues.

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But in the Bohr model, that's the model where he kind of

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modeled electrons as planets revolving around a star or

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around the Sun.

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And that model is actually useful, at least it's useful

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in my brain, to conceptualize the idea of energy states.

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So this is an electron around the nucleus, right?

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It's moving around in an orbit.

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And we know, and I want to emphasize, orbits aren't

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really what happen.

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Orbitals are what happen.

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And orbitals are more like probability functions as to

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where you might find the electron, while an orbit is a

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very kind of classical, mechanical way of describing

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the path of a classical object, like a

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planet around a star.

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I don't want to say the analogy too much.

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But if you view this model, the idea of energy levels

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start to make sense.

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For example, if I have something orbiting, if I have

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a planet orbiting a star, like that.

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And if it were to have more energy, perhaps its orbit

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would become more elliptical.

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Maybe for some reason I put some more energy into this.

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I had a little rocket booster on this planet right now that

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temporarily put some energy into it.

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Instead of going down this path, maybe it'll push it this

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way, and maybe it'll accelerate it

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a little bit faster.

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And maybe it'll go something like this.

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I don't know, I haven't done the math.

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But in general it's going to have a little bit higher

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kinetic energy, so it's going to get a little bit further

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away from the planet.

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And then maybe if I rocket-boosted it again, its

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path would look something like this.

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Its orbit would get further pushed out and as it

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approaches the planet, it actually would achieve faster

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speeds as it approaches the planet with gravity.

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And there's a couple of interesting things here.

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One, obviously, the planet or the rocket that has this orbit

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has more energy.

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This one right here will have more energy than, let's say,

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this one over here.

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And energy, even though we're talking in the quantum world

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and this is just analogy, because we know orbits don't

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really apply, but energy is really the same energy that we

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talk about in anything.

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And energy is the ability to do work or transmit heat or

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create heat.

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So, you know, if you're not doing work and you have

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energy, you might kind of waste the work

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by generating heat.

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We'll talk more about that in future videos.

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But it's the same idea, right?

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If I had a little rocket pack and put some energy into this,

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or pushed it somehow, I might get into this higher orbit.

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The idea of orbitals is the same thing, except obviously

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they aren't these well-defined paths.

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That as electrons get more energy, and that energy can be

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given to the electron, mainly through light waves, or

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electromagnetic waves can be put onto the electron.

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And when we do quantum mechanics, we'll do that in

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more detail.

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But, essentially, if you view light as a bunch of packets,

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as a bunch of photons, and a photon hits an electron in a

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certain energy state, all of a sudden it will enter a higher

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energy state.

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And maybe it'll go to this probably distribution that's a

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shell around that one.

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And maybe if, after it gets excited-- these are words that

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you hear physicists and chemists say a lot-- but

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excited just means that energy was put into the electron and

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it went to a higher energy state.

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And it might stay there or it might just want to go back to

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its lower energy state.

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So when it goes back to its lower energy state, it would

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emit the photon back, and that's actually why you see

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some things sometimes glow.

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But we'll talk more about that in the future, as well.

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But I really want to give this intuitive point, because in

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the rest of chemistry and in a lot of physics people talk a

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lot about energy states, or the electron going into a

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higher or lower energy state, and that's just the general

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idea, is that an electron in a kind of higher orbital has had

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energy put into it, although it wants to get back to its

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lower orbital.

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Now you might ask, how can an electron

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stay in a higher orbital?

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For example, what if an electron just stayed, what if

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we already had two electrons in this orbital over here?

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And we'll talk a little bit about how the different

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orbitals get filled.

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But I want to give you the intuition first. Let's say you

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had two electrons.

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They're just all over this place.

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You can't even pinpoint them.

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And then I were to add a third electron.

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So you might say, oh, the lowest energy state is this

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magenta inner sphere that I just drew.

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Why wouldn't that third electron go there?

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Well, my intuition is that, well there's already two

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electrons there, and although the electrons are attracted to

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the nucleus because the nucleus has all the positive

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charge in it, and the electrons have all the

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negative charge, it's repelled by these two electrons.

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Because negative, like charges repel each other.

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So it will want to stay away from these two electrons.

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And so it will go to the next energy state.

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It'll maybe go into this shell out here.

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And the other interesting thing about energy states--

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and this is key to chemistry when we start talking about

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reactivity and how something might react with something

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else, and why would it -- is that things at a high energy

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state, for example if we use the orbit analogy, this high

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energy state, in the case of planets they get further from

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the body that they're kind of attracted to, so the

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gravitational force is weaker.

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Or in the case of electrons, when they get further away

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from a high energy state, the coulomb

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force is weaker, right?

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The charges we talk about when we talk about electrons and

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protons, those are the coulomb forces.

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So this is a negative charge and then you have positive

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charges in the center.

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But it gets further away, I guess is the best way

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to think about it.

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And so the force from the nucleus is weaker, so they're

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easier to pluck off.

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They're easier to pluck off and maybe share with other

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atoms. Or maybe to give to other atoms, and we'll talk a

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lot about that when we talk about bonding.

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But I wanted to give you this intuition first.

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So then the next question that might arise is, well, so how

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do the electrons fill the different orbitals, and what

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do those orbitals actually look like?

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And I've cut and pasted some interesting

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graphics from Wikipedia.

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So here are the orbitals.

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Here are the different orbitals.

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And so there's two aspects to the orbital.

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One is its shell, its energy shell.

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And that's given by this number here, n.

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That's the energy shell.

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And just so you know, everything

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kind of fits together.

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Those energy shells correspond to periods in

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the periodic table.

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So a period on the periodic table is literally

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just a row in it.

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So this is period one in the periodic table, right there

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all the way to helium.

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That's period one.

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It's just the first row.

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And that means that the elements in that first period,

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that their electrons will fill the first energy shell.

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So for example, hydrogen has one proton.

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And everything we do, we're going to assume neutral atoms.

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So we can take-- we learned in the last video, that the

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atomic number tells you how many protons there are, right?

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This is how many protons there are in hydrogen.

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But if we assume it's a neutral atom, we can say that

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this is also the number of electrons.

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So we can use the atomic number also as an indicator of

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how many electrons in a neutral atom.

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So this has one electron.

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Where does it go?

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Well, it's in the first period, so it's going to go

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into the first energy shell.

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And so the first electron will go right here in

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the 1s energy shell.

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So if we wanted to write the electron configuration for

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hydrogen, we would write-- so hydrogen, the electron

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configuration, it's in the first energy cell, at 1s.

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And there's only one electron there.

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And what does that first orbital subshell, that

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s-shell, look like?

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It's just a sphere.

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It's actually what I just drew at the top of the video.

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It's literally just a sphere.

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And if I were to draw a cross-section of it, it gets

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denser in the center and then it gets less

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dense as you go outside.

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And in the last video I showed you what the helium, you could

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kind of say, the orbital function looks like.

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And you saw, it was really dark and dense in the middle

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and it got more sparse and grayer and

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whiter as you went outside.

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So what is helium's electron configuration?

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Well, in each of these subshells-- and I'll be a

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little bit more specific in probably the next video,

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because I'm pretty much out of time-- you can put two.

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I guess in each of the geometric configurations for

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each subshell, you can put two electrons.

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And we'll do that in some detail in the future.

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So the configuration for helium.

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It's in the first period.

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So it's 1s2.

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So in the s subshell within the first period or the first

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energy shall, it has two electrons there.

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Fascinating.

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So what about lithium?

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Lithium, right here.

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Also the name of an Evanescence song.

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I think it's the name of an Evanescence song because it's

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used to treat depression, or at least in the past it's been

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used to treat depression.

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So, lithium.

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What is its electron configuration?

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So the first electron goes into 1s1.

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The second electron goes into 1s2.

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And when I say the first or second, I'm

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saying energy states.

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So the first electron wants to go into the

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lowest energy state.

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That's in the s1.

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Then the second electron also wants to go there.

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And two electrons can fit in that first energy state, or

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that first sub-orbital, or that first shell.

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So then it becomes 1s2.

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Then lithium.

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It fills that first 1s2.

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It fills the first energy shell and that first subshell,

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which is the S-shape.

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And so now it has to go to the second energy shell, and that

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works out relative to what I told you before, because it's

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in the second period.

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The second period is that right there.

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Right?

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It's in the second period.

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So its electron configuration is going to be 1s2.

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Two of its electrons fill just the way helium filled.

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And then its third electron will be 2s1.

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So that's its electron configuration.

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What do I mean by 2s1?

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Well, so, lithium is going to have two electrons in that

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little dot that I overwrote it.

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And then around that dot, there's another shell, which

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is the second energy shell.

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And it's going to have one electron in there.

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So let me see if I can draw that.

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So it's going to have one probability, I guess, sphere,

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where the first two electrons are going to reside.

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And if this is a cross-section, that third

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electron is going to reside in it in a probability shell

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around that.

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When I draw these, that's not like the electron is exactly

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there in the orbital.

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I'm just drawing where you're just doing a cutoff, where you

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say it's a 90% chance of finding the electron.

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The electron could show up there or there or there.

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But this would be a very low probability, while right here

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would be a very, very high probability.

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Anyway, I'm out of time in this video.

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I'm going to continue this discussion in the next video.

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And I'll start talking about the more bizarro shapes the

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orbitals can take on, and maybe give you a little

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intuition on why these shapes aren't really that bizarro.

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