What is the oxidation number of the sulfur atom in Li2SO4 ? A) +4 B) +6 C) +2 D) -2
The answer is B) +6.
Since you're dealing with a neutral compound, the sum of the oxidation numbers of all the atoms that form said compound must be zero.
Since lithium is a group I alkali metal, its oxidation number will be +1. Oxygen, on the other hand, will have an oxidation number equal to -2. This means that you get
Alternatively, you can reason that, since lithium sulfate is an ionic compound, you can isolate the ions to get
Since sulfur is a part of the sulfate anion, which has a -2 net charge, you'll get