- [Instructor] Let's now see if we can 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 can 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 and that's because they have two electrons in their outermost shell and
they would like to lose them. And 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. And if you look at where bromine
sits in 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. And so bromine would
like 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 be to 1-. It's gonna wanna gain an electron, that's what the 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, well, we don't see any net charge here,
for an ionic compound, these things are going
to cancel each other out. The charge of the calcium cation is going to cancel out
with the bromide anion. So how is that going to happen? Well, have you 2+ here,
you only have 1- here, so you're gonna have to have two bromides for every of the calcium ions. So this is going to be, for
every one of the calciums, you're going to have two bromides. 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 2+, it's a Group Two element right over here. And bromine only gets a -1 or a 1- charge, so you're gonna need two of the bromides for every one of the calciums.