What is the molecularity of the following reactions?
$1.$ $NH_4NO_2(s) \rightarrow N_2(g) + 2H_2O(l)$
$2.$ $2HI(g) \rightarrow H_2(g) + I_2(g)$
$3.$ $2NO(g) + O_2(g) \rightarrow 2NO_2(g)$

(N/A) Molecularity is defined as the number of reacting species (atoms,ions,or molecules) taking part in an elementary reaction,which must collide simultaneously in order to bring about a chemical reaction.
$1.$ For the reaction $NH_4NO_2 \rightarrow N_2 + 2H_2O$,only one molecule of $NH_4NO_2$ is involved. Thus,it is a unimolecular reaction (molecularity = $1$).
$2.$ For the reaction $2HI \rightarrow H_2 + I_2$,two molecules of $HI$ are involved. Thus,it is a bimolecular reaction (molecularity = $2$).
$3.$ For the reaction $2NO + O_2 \rightarrow 2NO_2$,three molecules ($2$ of $NO$ and $1$ of $O_2$) are involved. Thus,it is a termolecular reaction (molecularity = $3$).