SO3
for the formal charges, we have:
for the single-bonded oxygens F.C. = 6 - [1/2(2) + 6] = -1
for the double-bonded oxygen F.C. = 6 - [1/2(4) + 4] = 0
for the sulfur F.C. = 6 - [1/2(8) + 0] = 2
Again, the formal charges add to zero. The oxidation numbers of O and S are -2 and +6, respectively. Note the three resonance forms!