This question is regarding polyatomic ions and their charge. I have always been told that I have to memorize the charge of polyatomic ions. The question is whether I can determine by myself the charge of two atoms when they join together to form a polyatomic ion. For example Se and O can form selenate SeO4, this polyatomic ion has a charge of -2. I would like to know why is the charge -2? But what I know the sum of all oxidation states of the joining ions must be equal to the net charge of the ion. But what happens then if I do not know the net charge of the ion? Hopefully these questions are not too silly. Thanks before hand, Aldo.