Of course, there was something we didn't show you in this picture. What happened to the line that connected the hydrogen atom and the oxygen atom? That line isn't just there to look pretty. It means something. It means that the hydrogen atom and the oxygen atom were covalently bonded to one another. And, of course, we know this means that the two atoms were sharing a pair of electrons. What happens to the pair of electrons?

It just so happens that the oxygen atom is going to win the custody battle over those electrons. The pair of electrons will stay with the oxygen atom when the hydrogen atom splits from the rest of the molecule. This may seem unfair, because out of those two electrons, one originally belonged to the hydrogen. So in effect, when the hydrogen leaves, it loses an electron, and the oxygen atom gains that electron.

The curved arrow shows where the electrons move. The electrons are drawn as little black dots. Remember that an electron has a negative charge. Since the oxygen atom now has an extra electron, it now has a negative charge. That's why the oxygen atom has a negative sign next to it. But what about the hydrogen atom? Normally, a free hydrogen atom would have one proton and one electron. But remember, it just lost its only electron. This means that the hydrogen atom, once it leaves the molecule, is nothing but a proton. You may remember that a proton has a positive charge. (That's why we draw the hydrogen atom with a positive sign next to it.)

So we're left with two things, with opposite charges. The hydrogen atom has a positive charge, and the remainder of the molecule has a negative charge. We call both of them ions. The hydrogen atom we call a hydrogen ion (duh!) and the rest of the molecule we would call, in this case, an acetylsalicylate ion.