We have seen that the charge of some ions can be determined from the periodic table.
However, for those elements that can form ions of more than one charge we need to look at the formula of the compound. Consider the formula NiCl2. Chlorine always forms a -1 ion (see above). Since there are two chlorines, the total negative charge in the molecule is -2. In order for the compound to be neutral (all compounds are), we must also have a total positive charge of +2. The nickel provides the positive charge and therefore the nickel ion must have a +2 charge.
What about Fe3(PO4)2? From our table of polyatomic ions we know that phosphate has a -3 charge. Since there are two phosphate ions, the total negative charge is -6. There must also be a total positive charge of +6. Since there are three iron ions, each must have a charge of +2.
