Identify the nonmetals in these compound: Mg0, NaH, AlB3, FeS

These compounds are all ionic structures, which means they're formed from one metal and one non metal ion. Usually, in the chemical formulas of these ionic compounds, the metal ion is in front of the non metal ion. When you identify the non metals (or metals), make sure you give their element name, instead of their ion name. (Eg in MgO, the ion names are magnesium and oxide ion, but the elements are magnesium and oxygen.)

Therefore, MgO, magnesium oxide, the non metal is oxygen.

NaH, sodium hydride, non metal is hydrogen.

For the third one, i suppose you're looking for AlBr3, because aluminum and boron are both metals and metalloids and they do not form an ionic bond. So, if you're looking for AlBr3, aluminum bromide, the non metal is bromine.

And for FeS, iron (ii) sulphide, the nonmetal is sulphur.