Polyatomic ions

Ions are atoms that are either missing electrons or have extra electrons. They are like atoms with a charge. Positive and negative charges attract each other. When they are pulled together, we get an ionic compound. Ionic compounds form large crystals with billions of ions.

In this case, each ion is made of just one atom. We say that they are atomic ions. They are not attached to each other by anything other than their electric charges. There is another way that atoms can bond to each other: forming molecules. Molecules are usually neutral, so they don’t attract each other nor form large crystals like the ions do.

But some molecules actually do have an electric charge. Since they are ions that consist of more than one atom, we call them polyatomic ions. This example is a charged molecule with one sulfur atom and four oxygen atoms. It’s the sulfate ion. The atoms in the sulphate ion are bonded together in the same way a molecule is.

The charge, in this case two minus, is distributed across the whole ion. These ions act just like the atomic ions when forming compounds. Since the sulfate ion has a negative charge It attracts positive ions. Here are some of the most common polyatomic ions: ammonium, with a plus one charge hydroxide and nitrate, each with the charge minus one Sulfate and carbonate, with a charge of minus two and phosphate with a charge of minus three. These ions often appear in salts or in solutions in the chemistry lab.

You can probably look them up every time you need them, but it’s a good idea just to learn them by heart. When these ions form compounds with other ions, there is a trick to writing the chemical formulae. Let’s say you want to write the formula for magnesium nitrate. It has twice as many nitrate ions as magnesium ions. But where should we put the number two?

This is not a very good spot, since it looks like we’ve got 32 oxygens. If you have more than one of any polyatomic ion, you have to put it in brackets, like this. There is another reason to know the formulae of some of the ions by heart. Many of the most common acids consist of one of these polyatomic ions together with hydrogen ions. The sulfate ion for instance forms sulfuric acid together with two hydrogen ions.

In the same way, nitric acid, carbonic acid, and phosphoric acid are simply the polyatomic ions nitrate, carbonate, and phosphate together with one, two, and three hydrogen ions. One more thing: did you notice how many hydrogen ions we needed to complete the formula of each acid? That’s right, an ion with a charge of minus three needs three hydrogen ions to form the corresponding acid. So if you know the formula of an acid, you immediately know both the formula and the charge of the polyatomic ion. And the other way around.