Oxidation States and Assigning Oxidation Numbers
It is important to know which atom is oxidized and which is reduced. Oxidation states or oxidation
numbers are assigned to atoms in order to keep track of the redistribution of electrons during a
redox reaction. In a redox reaction, the oxidation numbers of some atoms have to change to reflect
the gain or loss of electrons. By keeping track of and comparing the oxidation numbers of the atoms
on the reactant and the product side, it is possible to determine how many electrons are gained or
lost by each atom. The oxidation number of an atom in a compound is assigned according to the
following rules:
The oxidation number of free elements is zero. For example, the atoms in N 2 , P 4 , S 8 , and He all
have oxidation numbers of zero.
1.
The oxidation number for a monatomic ion is equal to the charge of the ion. For example, the
oxidation numbers for Na+, Cu2+, Fe3+, Cl−, and N3− are +1, +2, +3, −1, and −3, respectively.
2.
The oxidation number of each Group IA element in a compound is +1. The oxidation number
of each Group IIA element in a compound is +2.
3.
The oxidation number of each Group VIIA element (halogens) in a compound is −1, except
when combined with an element of higher electronegativity. For example, in HCl, the
oxidation number of Cl is −1; in HOCl, however, the oxidation number of Cl is +1 because of the
oxygen (see rule 6 below).
4.
The oxidation number of hydrogen is −1 in compounds with less electronegative elements
than hydrogen (Groups IA and IIA). Examples include NaH and CaH 2 . The more common
oxidation number of hydrogen is +1.
5.
In most compounds, the oxidation number of oxygen is −2. This is not the case, however, in
molecules such as OF 2 . Here, because F is more electronegative than O, the oxidation number
of oxygen is +2. Also, in peroxides such as BaO 2 , the oxidation number of O is −1 instead of −2
because of the structure of the peroxide ion, [O−O]2−. (Note that Ba, a group IIA element,
cannot be a +4 cation.)
6.
The sum of the oxidation numbers of all the atoms present in a neutral compound is zero. The
sum of the oxidation numbers of the atoms present in a polyatomic ion is equal to the charge
of the ion. Thus, for SO 4 2−, the sum of the oxidation numbers must be −2.