Let's think a little bit about
the molecule sodium chloride. If we look at the periodic
table right over here, we see that sodium
is a Group 1 element. It's an alkali metal. It has one valence electron. It's also not too
electronegative. It's sitting here on
the left-hand side of the periodic table. We know the general trend
for electronegativity is that it increases as
we go to the top right. So these elements
over here generally like to hog electrons. These elements
over here generally like to give away electrons. These are electropositive,
these are electronegative. So sodium right over here,
with its one valence electron, it's a very good candidate
for giving away an electron. On the other side of
the periodic table, we have chlorine
right over here. It's a Group 7 element. It's a halogen. It would love nothing more than
to gain an electron so that it can get to the magic number
of eight valence electrons. It is very electronegative. So what you can
imagine would happen is that if these two
things were to interact? The chlorine would nab sodium's
extra electron over here, and it would become
the chloride anion. And then sodium would
become the sodium cation. Cation just is a positive ion. Anion is just a negative ion. And so this one right
over here is now positive. Sodium is now positive. Chloride is a negative ion. And so they're going to be
attracted to each other, and they're going to
form this ionic bond. So this right over
here is an ionic bond. Now let's think about
something that's not ionic, where the electrons are
not being fully nabbed from one atom to another,
but they're being shared. And one of the most famous
examples of that is water. So we know water is H2O. Each water molecule is one
oxygen bonded to two hydrogens. And these two bonds
are covalent bonds. In each of these
bonds, you have a pair of electrons that
are being shared by both the hydrogen
and the oxygen. But we also know that this
isn't a completely equal sharing of the electrons. We look on the
periodic table here, oxygen is far more
electronegative than hydrogen is. And so because of that, the
electrons in these two bonds are going to spend
more time around oxygen than they are going to
spend around the hydrogens. And we've seen this before. This would give the oxygen
end of the water molecule a partially negative charge. That's the lowercase
Greek letter delta. We use that for the notation
of partially negative. And on the hydrogen
ends of the molecule, we're going to have a
partially positive charge. Now, this is the reality. But as we'll see later
on in future videos, it's sometimes inconvenient to
have this partial messiness. And so what I'm
going to do right now is introduce you to
what is fundamentally just an intellectual tool. It's just a convention
that chemists have invented that
allow us to get our heads around
a lot of reactions and allow us to think about how
is a reaction likely to occur. And that intellectual tool is
the idea of oxidation states. What the oxidation
state is, even if you're in a situation where you
have covalent bond, you say, well, look, I understand. Those are partial charges. These are covalent bonds. The electrons are being shared. But I don't like
this partial stuff. I want to just assume
hypothetically, what if these were ionic bonds? And you say, well, if these
had to be ionic bonds, then the oxygen would nab the
electrons from these pairs. And so the oxygen would have
a fully negative charge, a negative 2 charge. And the hydrogens would have
a fully positive charge each. And so, if we were to
write down the oxidation states for the atoms
in the water molecule-- let's write that
down, so H2O-- we would say that oxygen has an
oxidation state of negative 2, and each hydrogen atom has
an oxidation state of plus 1. And notice, the whole
molecule is neutral, and these things cancel
out with each other. Positive 1, positive 1,
that gets you to positive 2. Then you have negative 2. They cancel out. Now, the one thing, I keep
saying this is negative 2, but I wrote the
negative after it. If I wanted to write positive
1 as an oxidation state, I would actually write
it as 1 positive, although you can assume
that if someone just writes the positive. And this is just the
convention, to write the sign after the number when we are
writing actually ionic charges or oxidation states, because
an oxidation state is nothing but a hypothetical ionic charge. If you really had to-- if
you were forced to assume these aren't covalent bonds,
but these are ionic bonds. Once again, I want to stress. This is the reality. These are covalent bonds. These are partial charges,
the oxidation state, intellectual tool,
that's forcing us to pretend like
these are ionic bonds. And you might say well,
this kind of makes sense right over here. This involved
oxygen in some way. That's why it's called
oxidation states. And that's how I initially
conceptualized it when I first learned about this. You say, well, look,
each of these hydrogens lost an electron to oxygen. So it makes sense that we
say that each hydrogen got oxidized, so hydrogen
oxidized by oxygen. It makes sense that oxygen
would oxidize something else. This got done to it. The charge was taken away by
oxygen, so it got oxidized. Now, the other term on the other
side of oxidized is reduced. And the word "reduced"
really comes from the idea that oxygen's charge
has been reduced. So we could say,
O, or we could say oxygen has been reduced
by the hydrogens. And so there there's a
temptation here to say, well, OK, this must always
involve oxygen in some kind, because it seems to begin
with the same words. Well, that is not the case. Let's take, for example, if
this is an aqueous solution, hydrofluoric acid
right over here. You have a hydrogen covalently
bonded to a fluorine. Now, just like we saw
in water, fluorine is one of the most
electronegative elements. It's going to hog the electrons
in this covalent bond. So this is going to have a
partial negative charge here. This is going to have a
partial negative charge here. And this is going to have a
partially positive charge. But if we were going to think
about it in terms of oxidation states, we would say
when push comes to shove, if this had to be at an ionic
bond and not a covalent bond, what would be the charges
on each of these atoms? We can say, well, in
that case, hydrogen would lose an
electron, and it would have a full positive charge. And fluorine would
gain an electron and have a full negative charge. This is a hypothetical. Once again, the reality
is they're partial. It's a covalent bond. But the hypothetical one is
a full positive charge here and a full negative charge here. And so we would say
that the oxidation state in this molecule
for hydrogen is plus 1 and the oxidation state for
fluorine in this molecule is negative 1. And we would say that
hydrogen has been oxidized. And we could even say it's
been oxidized by the fluorine. And we would say
that the fluorine, because its hypothetical
ionic charge has been reduced, we would say fluorine
might be reduced. And you would say, wait, wait. Look, oh wow, we're
using the word "oxidize" even though there is no
oxygen to be seen here. And one way to think
about it, if someone had told that you had been
Bernie Madoffed, that doesn't necessarily mean that you
interacted with Bernie Madoff. It means that someone
did to you what Bernie Madoff would have done. Someone else-- someone
took your money, told you they were
going to invest it, and then put it
into a Ponzi scheme. Even if that person
is not Bernie Madoff, you could say that you
have been Bernie Madoffed. So here, fluorine
did to hydrogen what oxygen tends to do. It took an electron away. It oxidized the hydrogen. Now, that's how I
tend to remember it. If something has been oxidized,
it's losing an electron. And what I think about
it is, well, that's what oxygen would
have done to you. Oxygen is very electronegative. It tends to take electrons
away from other atoms. Now, there are other
mnemonics that you might see for remembering
what oxidation and reduction
actually represent. And I'll introduce those
to you, just because they might be helpful, and
they are introduced in a bunch of chemistry classes. One of the mnemonics is
LEO the lion says GER. I'll write says
in lowercase here, because it's really
not relevant. But LEO the lion says GER. And this is to remember that
losing an electron means you are being oxidized, or
losing electrons is oxidation. And gaining electrons
is reduction. So that's just a mnemonic. Another one that's
often used is OIL RIG. And this, essentially--
oxidation is losing electrons, reduction is gaining electrons.