- [Instructor] So we have the
formula for an ionic compound here, and the goal of this video is what do we call this thing? It clearly involves some
cobalt and some sulfur, but how do we name it? Well, the convention is, is
the first element to be listed is going to be our cation,
and if we look at cobalt over here, we see that
it is a D-block element and D-block elements are
tricky because you don't know exactly how it will ionize. So we know that this is
going to be our cation, it's going to be our positive
ion, but we don't know what the charge on each of those cobalt is actually going to be. So now let's look at the anion,
let's look at the sulfur, or as an anion, the sulfide. So let me underline that. And on the periodic table,
we see sulfur is out here that in its group, it would
want to gain two electrons in order to have a complete outer shell. It's just like oxygen, it
wants to gain two electrons. So the sulfide anion will look like this. So it will have sulfur when it ionizes will have a two minus
charge, just like oxygen, just like everything else in this group. It would want to gain
one, two electrons so that its outer shell looks
like that of a noble gas, looks like that of argon. We can use this as a clue
to figure out what must be the charge on the cobalts because we have three of the sulfides. Each of the sulfides
has a two minus charge, and we have three of them,
so that's going to give us a six minus charge all in. And then the cobalt, we have two of them. And so these two cobalt have to offset this six minus charge. They have to have a six plus charge. Well that means that each of them need to have a three plus charge. If each of these have a three
plus charge and you have two of them, then you're gonna
have six plus on the positive side and you're gonna have
six minus from the sulfides. And the reason why this is useful for us is now we can name this. We would call this ionic compound Cobalt III, cobalt and you would write it with Roman numerals
here, Cobalt III Sulfide, Cobalt III Sulfide. Now I know what you might be thinking. Hey, when we looked at
other ionic compounds, I didn't have to write the
charge of the cation there and the reason why the
convention is to do it here, and I don't have to write in upper case there, so let me rewrite it as cobalt (III) sufide. The reason why I wrote
that three in this case is because the cobalt can
take on multiple charges. It's known as polyvalent, polyvalent. If something is in group one,
you know it tends to have a positive one charge. Group two tends to have
a positive two charge. If it's a halide, it tends to
have a negative one charge. But these metals here
in this D-block here, they could be ionized in multiple ways. And so that's why we have to
figure out what the charge of the cobalt is, and we
write it here in the name. We would call this cobalt (III) sulfide. You could have other ionizations
of cobalt in other ionic compounds, but this one
is cobalt (III) sulfide. And you could go either way. You could say, okay if the
cobalt has a charge of three plus you could figure out its
formula right over here, or as we just did in
this video, you could go from the formula to the actual name.