Define Molecularity of a reaction. Discuss the molecularity of simple reactions.

According to the collision theory in order that a reaction may take place, the molecules must come close together and collide with one another.

Hence,
The number of atoms, ions or molecules that must collide with one another simultaneously so as to result into a chemical reaction is called the molecularity of the reaction.

Molecularity of Simple Reactions:
In elementary reactions or simple reactions, the molecularity is simply the sum of the molecules of the different reactants as represented by the balanced chemical equation. The molecularity of the reaction can be 1, 2, 3 etc and according to that the reaction will be known as unimolecular, bimolecular, trimolecular etc. For example:

Decomposition of O₂F₂: the balanced equation is:
O₂F₂ ————–> O₂ + F₂
Hence the molecularity of the above reaction is 1 and the reaction is called unimolecular.