What is molecularity of a relation ? Explain its types by examples.

The number of reacting species (atoms, ions or molecules) taking part in elementary reaction, which must collide simultaneously in order to bring about chemical reaction is called molecularity of a reaction. The reaction can be unimolecular, bimolecular and trimolecular.

$(a)$ Unimolecular reaction : The reaction can be unimolecular when one reacting species is involved. e.g. Decomposition of ammonium nitrite.

$\mathrm{NH}_{4} \mathrm{NO}_{2(\mathrm{~s})} \rightarrow \mathrm{N}_{2(\mathrm{~g})}+2 \mathrm{H}_{2} \mathrm{O}_{(\mathrm{g})}$

$(b)$ Bimolecular reaction : Bimolecular reactions involve simultaneous collision between two species. e.g. Dissociation of hydrogen iodide.

$2 \mathrm{HI} \rightarrow \mathrm{H}_{2}+\mathrm{I}_{2}$

$(c)$ Trimolecular or termolecular reaction : Trimolecular reaction involves simultaneous collision between three reacting species.

e.g. $2 \mathrm{NO}+\mathrm{O}_{2} \rightarrow 2 \mathrm{NO}_{2}$

The reaction with the molecularity three are very rare and slow to proceed.

(d) Polymolecular reactions : The probability that more than three molecules can collide and react simultaneously is very small. Hence, these are very rare$\&$ slow to proceed.