The guideline for determining the oxidation number of an element in a substance In free elemental substances(单质),each atom has an oxidation number being zero. 2For ions consisting of a single atom,the oxidation number is equal to the charge on the ion 3The oxidation number of hydrogen is normally +1,except when it is bounded to metals in binary (二元)compound.In these cases,its oxidation number is -1.The guideline for determining the oxidation number of an element in a substance : ①In free elemental substances (单质 ), each atom has an oxidation number being zero. ②For ions consisting of a single atom, the oxidation number is equal to the charge on the ion. ③The oxidation number of hydrogen is normally +1, except when it is bounded to metals in binary (二元) compound. In these cases, its oxidation number is –1