How To Calculate Oxidation Number?

Chemical reactions involve the transfer of electrons from one substance to the other. Atoms are reduced or oxidized when reactions occur. For a balanced chemical reaction, the mass and charge of reacting mixtures and the products are always conserved. The number that helps us to give the complete idea about the number of electrons gained or lost by an atom in a chemical reaction is known as oxidation number.

What is Oxidation Number?

The oxidation number of an atom is defined as the number that tells the number of electrons lost or gained by an atom of a particular element in the compound. Oxidation state tells us about the total number of electrons which have been added to the element or a negative oxidation state or removed from an element or a positive oxidation state to get its present state. Reduction results in the decrease in oxidation state whereas oxidation results in an increase in oxidation state.

Rules for assigning oxidation number:

  • While writing a chemical formula a common convention is used in which cation is written first followed by the anion. For example, in HCl, the H used is H+ whereas Cl is Cl–.
  • Free elements always have an oxidation state of zero. The atoms in N2 have an oxidation number of zero.
  • In the case of monatomic ions, the oxidation number equals the charge on the ion. For example, the oxidation number of N3- is -3.
  • Assigning oxidation number; oxidation number of group IA elements is +1, the oxidation number of group IIA elements is +2.
  • Sum of oxidation numbers of all the atoms present in a compound is equal to zero (0).
  • Some halogens (Cl, Br and I) have oxidation number -1 when they occur as halide ions in their respective compounds.
  • Some elements have the same oxidation number in different compounds.

Points to Remember

Whenever you are assigning an oxidation number to an element, make sure to know the following things beforehand.

  • Making sure the substance is elemental or not
  • Making sure the substance is ionic or not
  • If the substance is ionic, can we see any presence of monatomic ions
  • Which all elements follow specific rules
  • Which all elements don’t follow specific rules
  • In a polyatomic ion, the sum of oxidation numbers of all atoms = charge of the polyatomic ion
  • In a neutral compound, the sum of all the oxidation numbers of atoms or ions is 0

To reduce the possibility of finding out which element is more electronegative than the other element, a set of rules are being made to determine the oxidation number of an element in a compound. It is clear that non-metallic elements generally have negative oxidation numbers whereas metallic elements mostly have positive oxidation numbers.

Therefore, it’s clear that the oxidation number denotes the oxidation state of an element in a compound. To learn more about oxidation number, install Byju’s – the learning app.


Practise This Question

In the following reaction, SO2 acts as a reducing agent SO2+Cl2+2H2OH2SO4+2HCl . The equivalent weight of SO2 will be