Worked example: Finding the formula of an ionic compound | AP Chemistry | Khan Academy

Let's now see if we could come up with the chemical formula for the ionic compound calcium bromide. And like always, if you are inspired, pause the video and see if you could come up with it on your own.

All right, so the convention is that we write the positive ion first, and so that's a pretty good clue that calcium is going to be the positive ion. Now let's look at the periodic table to confirm that it's likely that calcium would ionize as a cation. Well, calcium is right over here in group two, and group two elements, also known as alkaline earth metals, they tend to ionize by losing two electrons. That's because they have two electrons in their outermost shell; they would like to lose them.

So when calcium ionizes, it is going to be—it is going to ionize as Ca²⁺. Now, let's look at the bromide part. The "ide" tells us that this is going to be a negative ion, or it's going to be an anion. If you look at where bromine sits at our periodic table, right over here, we see it is a halide. We see that it likes to gain an electron, and so it makes sense that it's going to be our anion.

Bromine is going to want to gain an electron to have eight electrons in its outermost shell. So our bromide anion is going to look like this: it's going to be one minus; it's going to want to gain an electron. That's what these elements in this group like to do.

Now, what is the formula going to be? And remember, the key here is for an ionic compound, especially one that has no net charge. Here for an ionic compound, we're going to have—these things are going to cancel each other out. The charge of the calcium cation is going to cancel out with the bromine, with the bromide anions.

So how is that going to happen? Well, you have two plus here; you only have one minus here. So you're going to have to have two bromides for every one of the calcium ions. So this is going to be—you're going to have two bromides for every one of the calciums. So it's going to be like this: Br₂.

And there you have it; that is the chemical formula for calcium bromide. And how did we know that we have two bromides for every calcium? Well, because when calcium ionizes, it's going to be two plus; it's a group two element right over here, and bromine only gets a negative one, or one minus charge. So, you're going to need two of the bromides for every one of the calciums.