Oxidation Number

Rusting of Iron Nails

Oxidation Number – is the charge that the atom of the element would have if complete transfer of electron takes place.
Oxidation number
(i) Free elements have an oxidation number of zero.
Na = 0
Mg = 0
C = 0
H₂ = 0
Br₂ = 0
(ii) Oxidation number of a simple monoatomic ions is the same as its charge.
Na⁺ ion = +1
Mg²⁺ ion = +2
O^2- ion = -2
Cl^- ion = -1
H⁺ ion = +1
(iii) Sum of the oxidation number for a neutral compound is zero.
CaH₂
(+2) + 2(-1)
= 0
Sum of oxidation number is 0
Al₂O₃
2(+3) + 3(-2)
= 0
Sum of oxidation number is 0
Iodine, Bromine, Chlorine, Nitrogen, Oxygen, Fluorine
—> Electronegativity increase
Cl₂O
2(+1) + (-2)
= 0
Sum of oxidation number is 0.
(Chlorine, bromine and iodine usually have the oxidation number of -1 except when combine with a more electronegative element.)
HClO
(+1) + (+1) + (-2)
= 0
Sum of oxidation number is 0.
(Chlorine, bromine and iodine usually have the oxidation number of -1 except when combine with a more electronegative element.)
(iv) Polyatomic ion, the sum of the oxidation numbers of all the atoms equals the charge on the ion.
SO₄ ^2-
(+6) + 4 (-2)
= +6 + (-8)
= -2
Sum of oxidation number is -2
Cr₂O₇^2-
2(+6) + 7(-2)
= -2
Sum of oxidation number is -2
(v) Calculating the oxidation numbers of elements in compounds or ions.
K₂Cr₂O₇
2 (+1) + 2x + 7 (-2) = 0
x = +6
Oxidation number of chromium in K₂Cr₂O₇ is +6
NO^3-
x + 3(-2) = -1
x = +5
Oxidation number of nitrogen in NO^3- is +5
Hydrogen peroxide, H₂O₂
2(+1) + 2x = 0
x = -1
Oxidation number of oxygen in H₂O₂ is -1 (and not -2)
(Usually oxidation number for combined oxygen usually is -2 except in peroxides)
F₂O
2(-1) + x = 0
x = +2
Oxidation number of oxygen in F₂O is +2 (and not -2)
(Usually oxidation number for combined oxygen usually is -2 except in fluorine compounds)
NaH
(+1) + x = 0
x = -1
Oxidation number of hydrogen in NaH is -1 (and not +1)
(Usually oxidation number for combined hydrogen usually is +1 except in metal hydrides)
AlH₃
(+3) + 3x = 0
x = -1
Oxidation number of hydrogen in AlH₃ is -1 (and not +1)
(Usually oxidation number for combined hydrogen usually is +1 except in metal hydrides)
MgH₂
(+2) + 2x = 0
x = -1
Oxidation number of hydrogen in MgH₂ is -1 (and not +1)
(Usually oxidation number for combined hydrogen usually is +1 except in metal hydrides)
(vi) Some metals show different oxidation numbers.

Compound	Oxidation number of manganese
MnSO4	+2
MnO2	+4
K2MnO4	+6
KMnO4	+7
Compound	Oxidation number of chromium
K2CrO4	+6
K2Cr2O7	+6

(vii) Usually non-metals have negative oxidation numbers but Cl, Br & I can have positive or negative oxidation number.

Compound	Oxidation number of chlorine
HCl	-1
ClO2	+4
HClO4	+7

Stay tune for the next installment in the “Oxidation and Reduction” series with focus on the difference between redox reaction and non-redox reactions.