Water has another great property—it can be ionized. Remember when we tossed sodium into water? We ended up with sodium ions in solution. But we also ended up with "water ions," as it were. When the sodium ionized, it did so by giving up the spare electron in its outer shell—we’ve seen this before. But that electron had to go somewhere. It went to a water molecule, where it broke the covalent bond between hydrogen and oxygen, by completing oxygen’s outer shell. Simply put, oxygen no longer needs a hydrogen to complete its outer shell. So we end up with a fragment of a water molecule called a hydroxyl ion, written OH-. It’s an ion because it’s got an extra electron and so it’s negatively charged. Note that we also end up with a hydrogen atom. But hydrogen doesn’t like to be all by itself—he’s got an outer shell to fill too, you know. So he binds to another hydrogen atom from another water molecule stripped by sodium, forming hydrogen gas. This forces us to balance the books by multiplying the entire reaction by two—for chemistry cannot create matter, and we must end up with the same atoms we started with, albeit in different combinations.. The point is this: we’ve created a new species, the hydroxyl ion, which is a highly reactive molecule and which we call a base. Throwing sodium metal into water results in a basic solution of sodium hydroxide.