Orbitals | Electronic structure of atoms | Chemistry | Khan Academy

Orbitals | Electronic structure of atoms | Chemistry | Khan Academy

November 6, 2019 100 By Kody Olson


In the video where we introduced
the atom, I went off a bit about how at the
center of an atom we have the nucleus, and it’s actually a
very small fraction of the total volume of the atom. And the electron, even though we
call it a particle, it can really be best described
as kind of a smear around this nucleus. That although it’s a particle,
because of the Heisenberg uncertainty principle, we can
never tell exactly at a given moment where the particle is
and what its momentum is. So to describe it as a particle
is a little bit, I don’t know, at best, it’s
a little bit strange. And we said that the way that
they describe it, they don’t say that this particle is in
an orbit, like the planets around the Sun in orbit
would be like that. That would be like the
orbit of Halley’s Comet around the Sun. Instead, it can be described
as a probability function around the nucleus. So if the nucleus is there, we
have one orbital, actually the 1s orbital, and we’ll talk
about that in this video. It’ll be a sphere around
the nucleus. And actually the sphere has
no strict boundary. Whenever you see someone draw
it, they’re just saying, where is 90% of the time the
electron going to be. And then they’ll cut
off a boundary. And they’ll say, OK, it’s going
to be within this sphere and it actually gets denser
as you get into the center of the sphere. So if this was a cross-section,
it would be really dense in the center, and
it gets less dense, less dense as you go outside. Which just means that there’s
a much higher probability of finding the electron in the 1s
orbital near the center than near the outside. Although this boundary point out
here is just artificial. You can find the electron
pretty much anywhere. It just has a much lower
probability out there than in here. But I’ll touch on that
in more detail in the rest of this video. But I wanted to go back
to the Bohr model. And the Bohr model is the kind
of– let me write that down. Bohr model. And sometimes it’s
nice to know it’s named after Niels Bohr. And don’t think that this
guy was some slouch. He was at the cutting
edge and this wasn’t even that long ago. This was roughly about
100 years ago. So already we’re talking about
things that you can probably dig up research papers in your
library not too long ago where people are debating some
of these issues. But in the Bohr model, that’s
the model where he kind of modeled electrons as planets
revolving around a star or around the Sun. And that model is actually
useful, at least it’s useful in my brain, to conceptualize
the idea of energy states. So this is an electron around
the nucleus, right? It’s moving around
in an orbit. And we know, and I want to
emphasize, orbits aren’t really what happen. Orbitals are what happen. And orbitals are more like
probability functions as to where you might find the
electron, while an orbit is a very kind of classical,
mechanical way of describing the path of a classical
object, like a planet around a star. I don’t want to say the
analogy too much. But if you view this model,
the idea of energy levels start to make sense. For example, if I have something
orbiting, if I have a planet orbiting a
star, like that. And if it were to have more
energy, perhaps its orbit would become more elliptical. Maybe for some reason I put some
more energy into this. I had a little rocket booster on
this planet right now that temporarily put some
energy into it. Instead of going down this path,
maybe it’ll push it this way, and maybe it’ll
accelerate it a little bit faster. And maybe it’ll go something
like this. I don’t know, I haven’t
done the math. But in general it’s going to
have a little bit higher kinetic energy, so it’s going
to get a little bit further away from the planet. And then maybe if I
rocket-boosted it again, its path would look something
like this. Its orbit would get further
pushed out and as it approaches the planet, it
actually would achieve faster speeds as it approaches the
planet with gravity. And there’s a couple of
interesting things here. One, obviously, the planet or
the rocket that has this orbit has more energy. This one right here will have
more energy than, let’s say, this one over here. And energy, even though we’re
talking in the quantum world and this is just analogy,
because we know orbits don’t really apply, but energy is
really the same energy that we talk about in anything. And energy is the ability to
do work or transmit heat or create heat. So, you know, if you’re not
doing work and you have energy, you might kind
of waste the work by generating heat. We’ll talk more about that
in future videos. But it’s the same idea, right? If I had a little rocket pack
and put some energy into this, or pushed it somehow, I might
get into this higher orbit. The idea of orbitals is the same
thing, except obviously they aren’t these well-defined
paths. That as electrons get more
energy, and that energy can be given to the electron, mainly
through light waves, or electromagnetic waves can be
put onto the electron. And when we do quantum
mechanics, we’ll do that in more detail. But, essentially, if you view
light as a bunch of packets, as a bunch of photons, and a
photon hits an electron in a certain energy state, all of a
sudden it will enter a higher energy state. And maybe it’ll go to this
probably distribution that’s a shell around that one. And maybe if, after it gets
excited– these are words that you hear physicists and chemists
say a lot– but excited just means that energy
was put into the electron and it went to a higher
energy state. And it might stay there or it
might just want to go back to its lower energy state. So when it goes back to its
lower energy state, it would emit the photon back, and that’s
actually why you see some things sometimes glow. But we’ll talk more about that
in the future, as well. But I really want to give this
intuitive point, because in the rest of chemistry and in a
lot of physics people talk a lot about energy states, or
the electron going into a higher or lower energy state,
and that’s just the general idea, is that an electron in a
kind of higher orbital has had energy put into it, although
it wants to get back to its lower orbital. Now you might ask, how
can an electron stay in a higher orbital? For example, what if an electron
just stayed, what if we already had two electrons
in this orbital over here? And we’ll talk a little bit
about how the different orbitals get filled. But I want to give you the
intuition first. Let’s say you had two electrons. They’re just all over
this place. You can’t even pinpoint them. And then I were to add
a third electron. So you might say, oh, the lowest
energy state is this magenta inner sphere
that I just drew. Why wouldn’t that third
electron go there? Well, my intuition is that,
well there’s already two electrons there, and although
the electrons are attracted to the nucleus because the nucleus
has all the positive charge in it, and the electrons
have all the negative charge, it’s repelled
by these two electrons. Because negative, like charges
repel each other. So it will want to stay away
from these two electrons. And so it will go to the
next energy state. It’ll maybe go into this
shell out here. And the other interesting thing
about energy states– and this is key to chemistry
when we start talking about reactivity and how something
might react with something else, and why would it — is
that things at a high energy state, for example if we use the
orbit analogy, this high energy state, in the case of
planets they get further from the body that they’re kind
of attracted to, so the gravitational force is weaker. Or in the case of electrons,
when they get further away from a high energy state,
the coulomb force is weaker, right? The charges we talk about when
we talk about electrons and protons, those are the
coulomb forces. So this is a negative charge
and then you have positive charges in the center. But it gets further away,
I guess is the best way to think about it. And so the force from the
nucleus is weaker, so they’re easier to pluck off. They’re easier to pluck off
and maybe share with other atoms. Or maybe to give to other
atoms, and we’ll talk a lot about that when we
talk about bonding. But I wanted to give you
this intuition first. So then the next question that
might arise is, well, so how do the electrons fill the
different orbitals, and what do those orbitals actually
look like? And I’ve cut and pasted
some interesting graphics from Wikipedia. So here are the orbitals. Here are the different
orbitals. And so there’s two aspects
to the orbital. One is its shell, its
energy shell. And that’s given by this
number here, n. That’s the energy shell. And just so you know,
everything kind of fits together. Those energy shells correspond
to periods in the periodic table. So a period on the periodic
table is literally just a row in it. So this is period one in the
periodic table, right there all the way to helium. That’s period one. It’s just the first row. And that means that the elements
in that first period, that their electrons will fill
the first energy shell. So for example, hydrogen
has one proton. And everything we do, we’re
going to assume neutral atoms. So we can take– we learned in
the last video, that the atomic number tells you how many
protons there are, right? This is how many protons
there are in hydrogen. But if we assume it’s a neutral
atom, we can say that this is also the number
of electrons. So we can use the atomic number
also as an indicator of how many electrons in
a neutral atom. So this has one electron. Where does it go? Well, it’s in the first period,
so it’s going to go into the first energy shell. And so the first electron
will go right here in the 1s energy shell. So if we wanted to write the
electron configuration for hydrogen, we would write–
so hydrogen, the electron configuration, it’s in the
first energy shell, at 1s. And there’s only one
electron there. And what does that first
orbital subshell, that s-shell, look like? It’s just a sphere. It’s actually what I just drew
at the top of the video. It’s literally just a sphere. And if I were to draw a
cross-section of it, it gets denser in the center and
then it gets less dense as you go outside. And in the last video I showed
you what the helium, you could kind of say, the orbital
function looks like. And you saw, it was really dark
and dense in the middle and it got more sparse
and grayer and whiter as you went outside. So what is helium’s electron
configuration? Well, in each of these
subshells– and I’ll be a little bit more specific in
probably the next video, because I’m pretty much out
of time– you can put two. I guess in each of the geometric
configurations for each subshell, you can
put two electrons. And we’ll do that in some
detail in the future. So the configuration
for helium. It’s in the first period. So it’s 1s2. So in the s subshell within the
first period or the first energy shell, it has two
electrons there. Fascinating. So what about lithium? Lithium, right here. Also the name of an
Evanescence song. I think it’s the name of an
Evanescence song because it’s used to treat depression, or at
least in the past it’s been used to treat depression. So, lithium. What is its electron
configuration? So the first electron
goes into 1s1. The second electron
goes into 1s2. And when I say the first
or second, I’m saying energy states. So the first electron
wants to go into the lowest energy state. That’s in the s1. Then the second electron
also wants to go there. And two electrons can fit in
that first energy state, or that first sub-orbital,
or that first shell. So then it becomes 1s2. Then lithium. It fills that first 1s2. It fills the first energy shell
and that first subshell, which is the S-shape. And so now it has to go to the
second energy shell, and that works out relative to what I
told you before, because it’s in the second period. The second period is
that right there. Right? It’s in the second period. So its electron configuration
is going to be 1s2. Two of its electrons fill just
the way helium filled. And then its third electron
will be 2s1. So that’s its electron
configuration. What do I mean by 2s1? Well, so, lithium is going to
have two electrons in that little dot that I
overwrote it. And then around that dot,
there’s another shell, which is the second energy shell. And it’s going to have one
electron in there. So let me see if I
can draw that. So it’s going to have one
probability, I guess, sphere, where the first two electrons
are going to reside. And if this is a cross-section,
that third electron is going to reside in
it in a probability shell around that. When I draw these, that’s not
like the electron is exactly there in the orbital. I’m just drawing where you’re
just doing a cutoff, where you say it’s a 90% chance of
finding the electron. The electron could show up
there or there or there. But this would be a very low
probability, while right here would be a very, very
high probability. Anyway, I’m out of time
in this video. I’m going to continue this
discussion in the next video. And I’ll start talking about
the more bizarro shapes the orbitals can take on, and
maybe give you a little intuition on why these shapes
aren’t really that bizarro.