 So let's predict the formula of these ionic compounds that are formed from the combination of the ions that are formed from these atoms, OK? So remember, what we learned from the periodic table is that the group numbers are very helpful in telling us what ion, these particular atoms, are going to form. So remember, if you don't have a periodic table that gives you the names, you're going to have to remember all of these elements. So sodium is Na, and oxygen is O. OK? So let's just write these elements first. Lithium? Well, let's do it this way. We'll write the elements, and then we'll write what charges you would expect them to have. So sodium's group 1, so you would expect it to have a positive 1 charge, and oxygen's group 16. So you would hope you would expect it to have a negative 2 charge, because it's too away from the noble gases. So remember, you've got to add these two charges up to make a compound. So plus 1, and they've got to be even. They've got to be equal to 0. Plus 1 minus 2 equals minus 1. So you're going to have to have 2 sodium plus 1 oxygen. And when you write the compound formula, it's Na2O. Like that. So when you combine them together, you're saying that there's two Na's for every one of these formula units. So let's do the same thing with lithium and bromine. So lithium is also a group 1 element. So LI is the atomic symbol. Group 1 is plus 1. Bromine is a halogen, so group 17. So it's going to have a minus 1, plus 1, and minus 1 add up to 0. So the formula unit for this one is lithium bromine, just LIBR. Aluminum, that prefers to have a plus 3 charge. And oxygen, we already know from the one before, prefers to have a minus 2 charge. OK, so if we do plus 3 minus 2, that's going to give us over here plus 1. So that's not going to work. So we've got to figure out some sort of number that these things have in common, 2 and 3. So what you can do is do 3 2's and 2 3's, and that would equal 6. So 3 times negative 2 is negative 6, and 2 times positive 3 is positive 6. And those cancel each other out, 6 and negative 6. So you don't write it like this, push together, but you write it with subscript. So it's going to be 8L2O3 is the formula unit for the compound that's made by combining aluminum and oxygen. And that barium and fluorine, barium is group 2. So it's going to have a 2 plus charge, fluorine, halogen, group 17, minus 1, plus 2, minus 1, don't equal 0. So we're going to have to put a 2 there to make the formula unit of this, which is going to be BAA2. And then the next problem, I've been wanting so bad to tell you what their names are, but that's the next problem. So let's get quick to that one, and I'll tell you the names. Because that would be the next thing you would have to do.