In which species does sulfur have the highest oxidation number? s8 (elemental form of sulfur) h2s so2 h2so3 k2so4?

There are rules to assigning conventional oxidation numbers to some elements. For those unspecified, you can solve them. This is how you solve it.

*S8. Since this is in elemental form, its oxidation number is assigned as 0.

*H2S. H is assigned with +1. Since the compound is neutral, the overall charge is 0. So,
2(+1) + x = 0
x = -2
The charge of S here is -2.

*SO₂. O is assigned with (-2). Using the same procedure,
x + 2(-2) = 0
x = +2
The charge of S here is +2.

*H₂SO₃.
2(+1) + x + 3(-2) = 0
x = +4.
The charge of S here is +4.

*K₂SO₄. K is assigned with +1. 
2(+1) + x + 4(-2) = 0
x = +6
The charge of S here is +6.

The S with the highest oxidation number is the one in K₂SO₄.

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