Chemistry, Third edition

OXIDATION NUMBERS

Rules for working out oxidation numbers

1.The oxidation number of an atom of a free elementiszero. For example the oxida-

tion number of nitrogen in N 2 or calcium in Ca is 0.

2.The oxidation number of the ion of an element is equal to its charge. For exam-

ple the oxidation number of copper in Cu^2 is2 and the oxidation number of

oxygen in O^2 is2.

3.Thealgebraic sumof the oxidation numbers of the atoms in the formula of an

electrically neutral compound is zero. For Ca(OH) 2 , for example, the sum of the

oxidation numbers is

[2 for Ca] [2(2) for O]  [2(1) for H] = 0

4.For convenience, shared electrons in covalent compounds are assigned to the ele-

ment having the greater electronegativity. For example, the oxidation number for

phosphorus in PCl 3 is3 (chlorine is 1), since phosphorus is less electro-

negative than chlorine.

5.Thealgebraic sumof the oxidation numbers of all the atoms of an ion is equal to

the charge on the ion. In SO 42 , for example, the sum of the oxidation numbers is

[6 for S] [4(2) for O] =  2

Remember that:

during chemical reactions an increase in oxidation number signifies

OXIDATION; a decrease REDUCTION.

103

Assigning

oxidation numbers

Assign oxidation numbers

to the underlined atoms in

the following examples:

(i) SO 2

(ii) NH 3

(iii) S 8

(iv) CrCl 5

(v) NO 3 

(vi) MnO 4 

(vii)XeF 2

(viii)Ca(VO 3 ) 2

(ix) BiO 3 

(x) Cr 2 O 72 .

Exercise 7C

Redox ladder for chlorine

Chlorine can have different oxidation states. Using the following species, construct a redox

ladder for chlorine:

Cl 2 , HCl, ClO 3 , ClO, Cl, HClO 4

Note that those species with the lowest oxidation number for chlorine should be at the

bottom of the ladder, whereas those with the highest oxidation number for chlorine are at

the top.

Exercise 7E

Oxidation ladders

SO 4 2– SO 3

SO 3 2– SO 2

S

H 2 S

Exercise 7D

Some elements have many oxidation states. You can construct redox

‘ladders’ like the one shown below for sulfur:

(i) What is the oxidation state of sulfur on each rung of the ‘ladder’?

(ii)SO 2 reacts with water to form the sulfiteion SO 32 . Is this a redox

reaction?

(iii)H 2 S reacts with SO 2 to precipitate sulfur:

2H 2 S(g)SO 2 (g)2H 2 O(l)3S(s)

Which species is reduced? Which species is oxidized?