Now that we know a little bit
about oxidation and reduction, what I want to do is
really just do an exercise to just make sure that we
can at least give our best shot at figuring
out the oxidation states for the constituent
atoms that make up a compound. So, for example, here
I have magnesium oxide, which is used in cement. It has other applications. And this is magnesium
hydroxide, which is actually used in antacids. It's used in deodorant. And what I want
you to think about, and I encourage you to
pause this video right now, is given these two molecules,
these two compounds, and what we know about
the periodic table, try to come up
with the oxidation states for the different
elements in each of these compounds. So I'm assuming that
you've given a go at it. Now let's try to
work through this or think through this together. So first of all, magnesium. Magnesium right over here. We see it's group two. It's an alkaline earth metal. It has two valence electrons. It's not that electronegative. We've already seen that
something in this group right over here with two
valence electrons, it's likely to give them away. So if it were to
form ionic bonds, or if it were to
be ionized, it's likely to lose two electrons. If you lose two electrons, you
would have a plus 2 charge. So magnesium would typically
have a plus 2 oxidation state. On the other side of the
periodic table, oxygen, group seven. It has six valence electrons. It's very electronegative, so
electronegative that oxidation is named for it. It likes to take electrons
from other elements. And oxygen in particular
likes to take two electrons. So it's not unusual to
see, actually anything in this group, but
especially oxygen, taking two electrons
from something else. If you take two electrons,
and you started off neutrally, or you started in
a neutral state, it's not unusual to see oxygen
at a negative 2 oxidation state. So given that, it seems
like this could work out. Magnesium could have a
positive 2 oxidation state. And actually when you write
it as a superscript here, the convention is to write
the positive after the 2. And oxygen would have or could
have a negative 2 oxidation state. And this makes sense relative
to the overall charge of the molecule. Positive 2 plus negative
2 is going to be 0. And that makes sense. This thing overall is
a neutral molecule. And not only in this
case is the oxidation state a hypothetical
ionic charge, if these were to be ionic
bonds, this actually is an ionic compound. Oxygen actually does
take two electrons. And magnesium actually does
give away two electrons. So in this case
the oxidation state is actually describing what
is happening ionically. Now let's think
about this one right over here, magnesium hydroxide. Well, just like before,
magnesium typically has an oxidation state, likes
to give away its electrons. So it could have an oxidation
state of positive 2, which would imply that the
entire hydroxide anion-- And let's just say
hydroxide for now. Well I'll say hydroxide anion. I kind of gave it away a little
bit-- that this hydroxide, or this part of the
molecule, the right-hand part of what I've written here, for
this whole thing to be neutral, it should have a negative
2 oxidation state. Now how does that make sense? Well we have two
hydroxides here. Notice this subscript
right over here. So if each of those hydroxides
has a negative 1 charge, or a negative 1, I guess you
could say, total oxidation state, then when you take
two of them together, they would net out
against the magnesium. And that does seem
to make sense. If oxygen has a negative
2 oxidation state, hydrogen has a positive
1 oxidation state. Each hydroxide part
of this molecule is going to have a net
oxidation state of negative 1. But then you have two of them. So the net oxidation
for this part of the molecule
or the compound is going to be negative 2 nets
out with the positive 2 from magnesium. So once again, it makes sense.