How does the oxidation number for sulfur change?

Question

S(s) + O₂(g) → SO₂(g)

Answer 1

S(s) does not have an oxidation number because only one kind of atom is present and there is no charge on the molecule, so the sulfur starts at 0.

When you add oxygen to make SO₂(g), the oxygen carries a charge of -2 in pretty much all cases, and you can tell that's the case here because it is positioned as the anion.

With two oxygen atoms carrying a charge of -2 you have a total charge of -4, so that means the sulfur is now at a +4 charge.

TLDR: S(s) charge=0 ----> SO₂ charge=+4