nomenclature

1. eg., the acid HClO4, we can either use "Perchloric acid" or "Chloric(VII) acid.
which is commonly use(how about the other similar compounds), Or SHOULD be use in the CE exam?

2. For the English and American spelling, Like the N2H4, Dinitrogen tetraoxide and Dinitrogen teroxide. So
which style of spelling should be use(in the CE exam or common situation)?

3. Hg2Cl2 is a covalent compound;
so how about Hg2(NO2)2