Determining Charge on the Metal Ion in an Ionic Compound

In order to properly name ionic compounds that contain a metal which can have more than one possible charge, you need to determine that charge on the metal ion. This can be done by remembering one simple rule. The total charge on any compound is zero.

Here's how we use this fact.

In the compound Cu₃PO₄, we know that the polyatomic ion PO₄ has a charge of -3.

To balance that charge (to end up with a total charge of 0) all of the copper ions together must total +_3. So, each of the copper ion must be +1. So this compound is copper I phosphate.

If you prefer to see the problem written out algebraically, it would look like this.

3(x) + -3 = 0

where x is the unknown charge on the copper ions.

In the compound Cu₃(PO₄)₂ we know that phosphate has a charge of -3 and that there are two of them. Therefore, the total negative charge is -6. So, the three coppers have to add together to make a +6 charge. Each of them must be +2. So this compound is called copper II phosphate.

Algebraically, that would look like this:

3(x) + 2(-3) = 0