Naming ionic compound with polyvalent ion | Atoms, compounds, and ions | Chemistry | Khan Academy

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 would we name it?

Well, the convention is that the first element to be listed is going to be our cation. If we look at cobalt over here, we see that it is a d-block element. 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. On the periodic table, we see sulfur is out here. 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, when it ionizes, sulfur will have a 2 minus charge, just like oxygen, just like everything else in this group. It would want to gain 1, 2 electrons so that its outer shell looks like that of a noble gas, looks like that of argon.

So 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, and each of the sulfides has a two minus charge, we have three of them. So that's going to give us a six minus charge all in.

Then, the cobalt—two of them—has to offset this six minus charge. They have to have a six plus charge. Well, that means that each of them needs to have a three plus charge. If each of these have a three plus charge and you have two of them, then you're going to have six plus on the positive side, and you're going to have six minus from the sulfides.

The reason why this is useful for us is that now we can name this. We would call this ionic compound cobalt(III) sulfide; and you would write with Roman numerals here: 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." The reason why the convention is to do it here—is actually, let me clarify—I don't have to write it in uppercase there, so let me rewrite it as cobalt(III) sulfide.

The reason why I wrote that three in this case is because the cobalt can take on multiple charges. It's known as 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 could be ionized in multiple ways, and so that's why we had 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 and other ionic compounds, but this one is cobalt(III) sulfide.

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 compound.