What atom/ion is being oxidized?

Question

S₂O₃^2- (aq) + 2 Br₂ (l) + 5 H₂O (l)  →→  2 SO₄^2- (aq) + 4 Br ^- (aq) + 10 H⁺ (aq)

Answer 1

Compare oxidation numbers on both sides of the equation and see which change.

S₂O₃^2- (aq) + 2 Br₂ (l) + 5 H₂O (l)  →→  2 SO₄^2- (aq) + 4 Br ^- (aq) + 10 H⁺ (aq)

+2..-2.................0.............+1..-2...............+6..-2................-1...................+1..........

So, S goes from +2 in S₂O₃^2- to +6 in SO₄^2- so it has been oxidized