What is the oxidation number of S in Li2SO4?

S = + 6

The oxidation number of a free element is always 0. The oxidation number of a monatomic ion equals the charge of the ion. Fluorine in compounds is always assigned an oxidation number of - 1. The alkali metals (group I) always have an oxidation number of + 1. The alkaline earth metals (group II) are always assigned an oxidation number of + 2. Oxygen almost always has an oxidation number of - 2, except in peroxides (H2O2) where it is - 1 and in compounds with fluorine (OF2) where it is + 2. Hydrogen has an oxidation number of + 1 when combined with non-metals, but it has an oxidation number of - 1 when combined with metals. The algebraic sum of the oxidation numbers of elements in a compound is zero. The algebraic sum of the oxidation states in an ion is equal to the charge on the ion.