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.

7.