Question

which electrons are the valence electrons of the elemental vanadium, elemental selenium and the sulfur atom in a sulfate ion?

My question is why does sulfur in sulfate ion have 12 valence electrons? The original is 6.

Answer 1

To answer this question, you ask yourself why do atoms combine in the first place?!

Obviously, atoms combine with each other either by transfer of electrons(resulting in the formation of ionic bond) or by sharing of valence electrons (resulting in the formation of covalent bond) in order to have an octet of electrons in their valence shells.

Now sulfur atom belongs to the third period of periodic table. This means sulfur atom has low energy 3d orbitals which it can use to form bonds with other atoms. This implies sulfur atom has more than 4 valence shell orbitals (one 3s, three 3p and five 3d) to use in bond formation. So clearly sulfur atom can contain more than 8 electrons in its valence shell(it always does not!). In sulfate ion, sulfur has 6 valence electrons of its own, 2 electrons contributed by two of the oxygen atoms which form double bonds and 2 electrons by other two oxygen atoms which form single bonds(because each boundary atom contributes one electron per bond to the valence shell of the central atom).

Thus total number of valence electrons in S = 6 + 2×2 + 2×1 = 12 electrons.