The formulas for compounds formed from these pairs of ions are as follows:
1. Ba2+, S2-
BaS, 1 atom of Ba2+ is needed to have a stable compound with S2- .
2. Li+, O2-
Li2O, 2 atoms of Li+ is needed to have a stable compound with O2- .
3. Ca2+, N3-
Ca3N2, 3 atoms of Ca2+ is needed to have a stable compound with 2 atoms of N3- .
4. Cu2+, I-
CuI2, 1 atom of Cu2+ is needed to have a stable compound with 2 atoms of I- .
