Oxidation number, Rules governing oxidation number

1. Standard/ Class/ Grade XI Chemistry
Chapter 5 Redox Reactions
Gurudatta K Wagh, gkwagh@gmail.com
Oxidation Number
Ionic compounds are formed by the loss or gain of electrons.
Oxidation and reduction play an important role in the formation of
ionic compounds.
When molecular compounds or covalent compounds are formed
there is sharing of electrons. There is no loss or gain of electrons.
Oxidation number is introduced to understand the role of oxidation
and reduction. Oxidation number does not always mean ionic charge.
In monoatomic ions such as Cl- the oxidation number is -1, in Cu2+ it is
+2.

2. In polyatomic molecules or ions the oxidation number is assigned
arbitrarily. For example in case of carbon monoxide, the oxidation
number of oxygen atom is -2.
Oxidation number also called oxidation state of an atom is defined as
the number of charges it would carry if electrons were completely
transferred.
The oxidation state, often called the oxidation number, is an indicator
of the degree of oxidation (loss of electrons) of an atom in a chemical
compound. It may be positive, negative or zero. It is a hypothetical
charge.

3. To deal with the hypothesis of oxidation number certain rules have
been framed.
• The oxidation number of an atom in free uncombined state is zero.
Ca, H2, O3
• The oxidation number of an atom in a monoatomic ion is equal to
its charge.
Ba2+ = +2, Cr3+ = +3, K+ = +1, Br- = -1, S2- = -2
• The oxidation number of an atom of H is +1 or -1.
Hydrogen with non-metals is assigned oxidation number +1.
Hydrogen with metals is assigned oxidation number -1.
[O-H]- Oxygen -2, Hydrogen +1
H-O-H Hydrogen +1, Oxygen -2
Li-H Lithium +1, Hydrogen -1
H-Ca-H Hydrogen -1, Calcium +2

4. • The oxidation number of oxygen is usually -2. But in peroxides and
peroxide ions it is -1.
Ca-O Calcium oxide Calcium +2, Oxygen -2
H-O-O-H Hydrogen peroxide Hydrogen +1, Oxygen -1
[O-O]2- Peroxide ion Oxygen -1
OF2 Oxygen difluoride Oxygen +2, Fluoride -1
• The oxidation number of F, Cl, Br, I is -1. But when Cl, Br, I are
bonded to oxygen the oxidation number is +1.
H-F Hydrogen fluoride Hydrogen +1, Fluorine -1
K-Br Potassium bromide Potassium +1, Bromine -1
Cl-O-Cl Dichlorine monoxide (Cl2O) Chlorine +1, Oxygen -2
H-O-Cl Hypochlorous acid (HClO) Hydrogen +1, Oxygen -2,
Chlorine +1

5. • The algebraic sum of the oxidation states of all the atoms in a
neutral molecule is zero.
CdS Cadmium sulphide +2 -2 = 0
• The algebraic sum of the oxidation states of all the atoms in a
polyatomic ion is equal to the net charge of the ion.
SO4
2- = +6 + 4 X (-2) = 6-8 = -2

6. Oxidation number
• may be fractional
• is positive in metallic elements
• positive or negative in non-metallic elements
• is +1 in alkali metals
• is +2 in alkaline earth metals
Although electrons cannot be transferred in fraction, it is seen that in
a compound or ion it is found to be fractional. Fractional oxidation
number is the average fractional number e.g. S4O6
2- tetrathionate ion
(Figure). The oxidation number of end S atoms is +5 each and of the
middle atoms is 0 each. The total oxidation number of 4 S atoms is
5+0+0+5=10 and the average oxidation number is 10 ÷ 4 = 2.5.
Oxidation number is represented in Roman numbers in parenthesis
(brackets) after the symbol of the metal e.g. Au(III)Cl3 auric chloride,
Sn(II)Cl2 stannous chloride, Hg(II)Cl2 mercuric chloride.
Tetrathionate ion