Odd-odd-odd rule for polyatomic ions
by Dr. Arno Papazyan

Here is a “trick” to predict the charges on polyatomic ions if you forget them. All you need is a periodic table. It's based on bonded atoms’ strong preference for even number of electrons.

If the polyatomic ion contains an odd number of atoms from elements with odd atomic number the charge of the polyatomic ion is odd

For the common ions encountered in General Chemistry:

If the predicted charge for a polyatomic anion is odd, it is almost always -1
(except -3 for phosphate and citrate)

If the predicted charge for a polyatomic anion is not odd, it has to be -2

Very few polyatomic cations are encountered in General Chemistry. If the predicted charge for a polyatomic cation is odd, it is +1; if it is not odd, it has to be +2

Just try it on any of the polyatomic ions!

You just eliminated the need to “memorize” the charges of polyatomic ions. Practice enough so you do remember most of them, but it's good to have a way to predict them if you forget them.

In the examples below, following elements have odd atomic numbers
H: atomic no.=1     N: atomic no.=7      Cl: atomic no.=17    Mn: atomic no.=25