Explain why ionic compounds are formed when a metal bonds with a nonmetal but covalent compounds are formed when two nonmetals bond.

Two nonmetals have enough electrons to share some for completing, so the bonds are covalent. A metal doesn't have that much, so it gives the few electrons on its outer layer more easily than a nonmetal (there must be a nonmetal to get them). This bond is ionic.
I'm still unaware of possible bonds between 2 metals (are there any? :) )

Because electrons have negligible mass, to account for the mass of the isotope, there must be 16 neutrons, 16 neutrally charged, massive, fundamental particles present in the sulfur nucleus. and thus finally, if we have the 32s2− ion, there are 18 electrons, 16 protons, and 16 neutrons.