Nonmetals tend to form what ions?
Negative ions with a charge equal to the number of electrons required to complete their octet of valence electrons.
As a result, they tend to take place in reactions that make it possible for these atoms to acquire one or more electrons. The high electron affinity persists until the valence shell is full.
So, elements in column 17 (halogens) form ions of charge -1.
Elements in column 16 form -2 ions
Elements in column 15 that are non-metals (not all of them are!) form -3 ions.