Example naming ionic compound

Let's get some practice naming ionic compounds. I have a formula for an ionic compound right over here, but how would I say this? If you get inspired, pause the video and try to work it out on your own.

Well, we could see that it has some magnesium, and it has some phosphorus. The convention is that the positive ion is written first. So if this follows the convention, the magnesium is going to be our cation. When you look at magnesium in the periodic table right over here, it makes sense that it is going to be the cation because it is a Group Two element, an alkaline earth metal. When those get ionized, they tend to lose two electrons; they have two electrons in their outermost shell, and so when they ionize, they tend to lose those.

So, when magnesium ionizes, it tends to be magnesium 2+. Each of these three magnesium ions is probably magnesium 2+. If we want to see what is going on in this compound, we could write it like this, which isn't what you would typically see, but this is just to see what the constituent ions of this compound actually are.

Now, as I mentioned, the cation tends to be written first, so that's the magnesium, and the anion tends to be written second, which is the phosphorus here if we follow the convention. So, that's the phosphorus right over here. Does it make sense that phosphorus would be an anion? Well, phosphorus is out on the right-hand side of the periodic table. In order for it to complete its outermost shell, it wants to gain one, two, three electrons.

So when it ionizes, it makes sense that it gains those three electrons, so that phosphorus becomes phosphide 3-. These two phosphides in this formula we could write like this, so we have two phosphide anions. We could just write it like this.

Now, the whole reason why I wrote out the constituent ions like this is to make sure that it'll all end up being neutral. The three magnesium ions will have a collective six plus charge, and the two phosphides are going to have a collective six minus. When you add the six pluses and the six minuses, they are all going to cancel out.

Let me write this: you have 6+ and then you have 6-, which once again makes us feel good that we're understanding this ionic compound well. An ionic compound didn't write any net charge here, so this is going to be neutral. The cations and the anion are going to cancel out.

But once again, what do we call this thing? Well, the convention is we just write the elemental name or we say the elemental name for the cation. So this is going to be magnesium. Magnesium, and then we say the I-verion of the anion. If this is phosphorus, we say phosphide.

So, this is going to be magnesium phosphide. Actually, I don't have to capitalize it, so let me write it all lowercase: magnesium phosphide. Now, you might be saying, "Hey, this doesn't have a lot of information in it. How do I, if someone just told me magnesium phosphide, how do I construct what we originally started with?"

Well, the answer is you would have to say, "Okay, magnesium is our cation; it's right over here. It's group two; it will have a positive two charge when it ionizes." So you would look at that and say, "Okay, phosphide. Well, that tends to have a three minus charge when it ionizes," which we saw over here.

Then you'd say, "Okay, well, what ratio would I have to have between these two things in order for them to cancel out?" Well, if for every three of these magnesiums I have two phosphides, then they're going to cancel out. So that's how you could go from the name to the actual formula, which we will actually do in the next video.