If an element is by itself like Al(s) then it\'s oxidation number is zero. Elements in their elemental state always have an oxidation number of zero, but you have to know the elemental state of certain elements. Like Oxygen, the elemental state of oxygen is O2 because it is diatomic. Therefore whenever you see O2 you know it has an oxidation number = 0. They will have a positive or negative number if they are in ion, like Ca2+ has an oxidation number = 2+ When elements are in a compound there are certain rules that apply to determining their oxidations number, but it sounds like you\'re simply referring to a single element alone and not in a compound. Hope that helps! Solution If an element is by itself like Al(s) then it\'s oxidation number is zero. Elements in their elemental state always have an oxidation number of zero, but you have to know the elemental state of certain elements. Like Oxygen, the elemental state of oxygen is O2 because it is diatomic. Therefore whenever you see O2 you know it has an oxidation number = 0. They will have a positive or negative number if they are in ion, like Ca2+ has an oxidation number = 2+ When elements are in a compound there are certain rules that apply to determining their oxidations number, but it sounds like you\'re simply referring to a single element alone and not in a compound. Hope that helps!.