The oxidation number of an element in its free, uncombined state is

Explanation

An uncombined element has not gained or lost any electrons, so its oxidation number is zero, and this holds for atoms such as helium and for molecules such as O2, P4 and S8 alike. This rule is the starting point for every oxidation number calculation and for recognising which species has been oxidised in a reaction. The group number gives the usual oxidation state in compounds, not in the free element.