What types of atoms usually become negative ions?
Non-metals come the right hand side of the Periodic Table, i.e. the right hand side of the Periodic Table as we face it.....(I will get sick of qualifying this one day!).
Because we add electrons to the same valence shell, and we also add protons to the nucleus, atoms in general undergo a contraction in atomic radius, with the smallest atoms to the right of the Periodic Table (again, as we face it, but I think you know what I mean).
And thus nitrogen, oxygen, fluorine, chlorine, etc. TEND to form negative ions, i.e.
On t'other hand, metals tend to be reducing; they tend to donate electrons to an oxidant..... Capisce?