Valency and Oxidation States of Elements

The differencences between valency and oxidation state

Valency Oxidation state/ number
It is combining capacity of an atom. It is the number of electrons that an atom can gain, lose or share to achieve a stable configuration. The number of electrons a particular element has lost, gained or shared on forming a compound.
It is determined by electrons in the outermost shell of an atom (valence electrons). Or measure of the number of bonds an atom can form. It is determined by gain (negative) or loss (positive) of electrons in a chemical reaction or compound. Or Measure of degree of oxidation or reduction of an atom.
Its values are usually positive integers. It can be positive, negative, zero even fraction intergers.
It is fixed for a given(particular) element. It can vary depending on the compound or reaction the element is involved.
It is a property of individual atoms or elements and is not affected by the presence of other atoms or compounds. It is a property of atoms within a compound or ion. It is not solely limited to an individual atom, but encompasses the electron distribution within a compound or ion (valence and non-valence electrons.)
For a compound an oxidation state is usually dependent on the oxidation state of the other element in a compound. It can be either positive or negative.

General Rules for determining oxidation state of an element in a compound

First write the oxidation states of each element in a compound excluding the one you want to determine. Then consider the following rules:
  1. The oxidation state of an atom in its elemental state is always zero. For example these elements or molecules have zero oxidation states. N2, P4, K, S8, Ca etc.
  2. The oxidation state of any monatomic ion is equal to its charge. For example, Na+=+1, Ca2+=+2.
  3. The sum of the oxidation state of all atoms in a radical, is equal to the charge on that radical.

    4. NH4+ =+1, SO42-= -2
  4. The sum of the oxidation numbers in a neutral molecule is zero. For example, NaCl=0, HCl=0
Some oxidation state can be obtained from the periodic table as follows:
Group I II III IV V VI VII VIII
Oxidation state. +1 +2 +3 +4 -3 -2 -1 0

VARIABLE OXIDATION states

Some elements have variable oxidation states, examples of these elements are:
Element Oxidation numberCommon name
Copper, Cu +1 Copper (I)
+2Copper (II)
Iron, Fe +2 Iron (II)
+3 Iron (III)
Lead, Pb +2 Lead (II)
+4Lead (IV)

Worked examples

  1. Find the oxidation state of chlorine in KClO3
    Soln.
    Oxidation states of K=+1, O=-2, Cl=?
    Total oxidation states of a molecule as stated in rule no. 5 is zero.
    Therefore, KClO3=0
    Then, KClO3
    +1 +Cl + (-2×3)= 0
    1+Cl - 6=0
    Cl-5=0
    Cl=+5
    The oxidation state of chlorine in KClO3 is +5.
  2. Find the oxidation state of sulphur in SO42-
    Oxidation state of O = -2, S=?
    S + (-2×4)=⁻2 (rule number 4)
    S-8=-2
    S=-2+8
    S=+6.

Special Case: The Unusual in Oxidation States

For elements combined with a radical within which there is element exhibiting variable oxidation state:
Examples
  1. Calculate the oxidation state of aluminium in Al2(SO4)3
    Solution
    Calculate the oxidation state of Sulphur in SO42-
    You will get +6
    Then use this to calculate the O.S of aluminium:
    2Al + (+6x3) + (-2 x 4 x 3) = 0
    2Al + 18 + (-24) = 0
    2Al - 6 = 0
    2Al = +6 
    2Al =  +6
 2      2
  = +3
    The oxidation state of aluminium is +3

  2. Calculate the oxidation state of copper in CuSO4
    3. Solution
    Cu + (+6) + (-2 x 4) = 0
    Cu - 2 = 0
    Cu = + 2
    The oxidation number is + 2.

Alternative Method

Also you can solve these unusuals by treating the radical as a single element. So use the oxidation state of the radical as it is and calculate the O.S of the other.

Al2(SO4)3
Al =? SO4 = -2
therefore,
2Al + (-2 x 3) = 0
2Al - 6 = 0
2Al = 0+6
2Al = +6
Al = +3

The chemical formulae

Chemical formula is a representation of a compound or molecule using symbols and numbers.

How to write a chemical formula

  1. Write the appropriate symbols for the elements or radicals involved. The symbols for the metal or positive radical usually come first.
  2. Write valencies superscript and on the right side of the symbol of an element/radical.
  3. Write the symbols again, reversing the valencies and write the valencies as subscript on the right side of the symbol of element or radical.
  4. The valency of 1 is simply assumed and not written in a chemical formula.
  5. Simplify if necessarily. But the simplification should involve the valencies exchanged only by using the greatest common factor.

Worked examples

  1. Write the chemical formula of calcium oxide.
    Soln.
    Rule 1: Chemical symbols
    Ca O
    Rule 2: Valency of both Ca and O as superscript.
    Ca2 O2
    Rule 3. Exchanging the valencies and writing them as subscript
    Ca2 O2
    Rule5: Simplifying by dividing by two both side
    CaO is the final formula.

  2. Write chemical formula of aluminium chloride.
    Soln.
                   Al3       Cl1
                    ⤩
               Al1       Cl3
    The chemical formula is AlCl3
The number one has been assumed in the above case.

Practice the following

Write the chemical formulae of the following compounds
  1. Sodium chloride
  2. Sodium sulphate
  3. Potassium hydroxide
  4. Zinc carbonate
  5. Lead (II) chloride
Form Two topics