Can you explain the difference between the charge of an ion such as a charge of 2 for an O2 minus ion and the oxidation state of an atom such as the plus 4 oxidation state of carbon in CH4.?

Answer 1

Charge is exactly what it says, the number of positive or negative charges carried by a species, so as you say the oxide ion carries a charge of 2-. Oxidation state is a more general idea which helps us when thinking about redox. It is the charge which the atom would carry if the bond were ionic. Many bonds are not ionic, but covalent, yet one partner in the bond may be considered more positive and the other more negative. For instance, in the permanganate ion, MnO4- (where the 4 should be a subscript and the minus sign a superscript) the manganese is bonded with a lot of oxygen. If the internal bonds were ionic, the manganese would carry a charge of +7. In MnO2, it would only carry +4, so we say the manganese is less oxidised in MnO2. Oxidation state is the formal charge carried by an atom. It can be used for either ionic or covalent compounds and helps us to keep track of what electrons are doing in a reaction. If the compound contains only simple ions it's the same thing as the charge, but if we are looking at a covalent substance, (and remember that polyatomic ions like permanganate contain covalent bonds within them), then it's only an idea in our heads. Note that it's usual to put the sign first for an oxidation number and second for a charge. Thus we say the charge on an oxide ion is 2-, but it's in the -2 oxidation state.