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 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