Assigning Oxidation Numbers
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Assigning oxidation numbers means applying a fixed priority of guidelines in order — free elements are zero, then fixed values for a handful of common elements, then whatever value balances the formula's already-known overall charge.
In sulfate, SO4 2−, oxygen is assigned its usual −2, giving four oxygens a combined charge of −8. Since the ion's overall charge is 2−, sulfur's oxidation number must balance that difference: sulfur is +6, because +6 plus −8 equals −2, matching the ion's actual, known charge exactly, with sulfur's value found last rather than assumed first.
Assigning oxygen's or hydrogen's usual value without checking whether the guideline's own exceptions apply, such as oxygen in a peroxide, produces a number that fails to balance the formula's real charge.