Chemistry question on compounds and ions?

[Miss Julia]

For the following pairs pf ions, use the concept that a chemical compound must have a net charge of zero to predict the simplest compound that the ions are most likely to form.
a) Cr^+3 and S^2-
b) Cr^2+ and O^2-
Al^2+ and F^-
d) Al^3+ and O^2-
e) Al^3+ and P^3-
f) Li^+ and N^3-
Please help, I wouldn't mind a little explaination either. Thanks, I will give ten points for best answer..