There are rules we can memorize...
1. Ionic compounds are always written in “first name/last name” format; that is the cation (first name) is written first, followed by the anion (last name).
For example, in CaH2, Ca is the “first name” and, therefore, is the cation, Ca2+ ; H is the “last name” and, therefore, is anion, H- (you need two of them for charges to balance.
2. Elements in their natural state have an oxidation number of 0. No exceptions!
For example, the atom in Zn and the diatomic Cl2 both have oxidation numbers of 0.
3. Monatomic ions have oxidation numbers equal to the charge...