"oxidation" refer to reactions in which an atoms loses or gains electrons.An oxidation state is a number that is assigned to an element in a chemical combination.A lot of oxidation number calculation is memorization. Some elements have fixed oxidation numbers eg, oxygen O2-, hydrogen H+, sodium Na+.Transition metals can have more than one oxidation number, for example, sulfur S 2-, 4+, 6+ .
For sulfate (SO4 2-), the S atom is in the 6+ oxidation state. Each oxygen atom (O) has an oxidation state of 2-. So, 4 x 2- = 8-. The overall charge on the sulfate ion is 2-, so the oxidation state on S must be 6+ (8- + 6+ = 2-).