What is the molecularity of a reaction? Explain its types with examples.

(N/A) The molecularity of a reaction is defined as the total number of reacting species (atoms,ions,or molecules) that must collide simultaneously in an elementary reaction to bring about a chemical change.
$(a)$ Unimolecular reaction: Involves only one reacting species.
Example: Decomposition of ammonium nitrite: $NH_4NO_2(s) \rightarrow N_2(g) + 2H_2O(g)$
$(b)$ Bimolecular reaction: Involves the simultaneous collision of two reacting species.
Example: Dissociation of hydrogen iodide: $2HI(g) \rightarrow H_2(g) + I_2(g)$
$(c)$ Trimolecular (or termolecular) reaction: Involves the simultaneous collision of three reacting species.
Example: $2NO(g) + O_2(g) \rightarrow 2NO_2(g)$
Reactions with molecularity greater than three are very rare because the probability of simultaneous collision of more than three molecules is extremely low.