Q.A certain reaction occurs in the following steps —

Each step involves 2 reacting species (bimolecular); $ClO$ is generated then consumed (intermediate); adding the steps and cancelling $Cl$/$ClO$ gives the overall reaction.

1. Molecularity: molecularity is the number of reactant species that must collide simultaneously in an elementary step. Step (i): $Cl_{(g)} + O_{3(g)} \rightarrow ClO_{(g)} + O_{2(g)}$ — two reactant species ($Cl$, $O_3$) collide $\Rightarrow$ bimolecular (molecularity = 2). Step (ii): $ClO_{(g)} + O_{(g)} \rightarrow Cl_{(g)} + O_{2(g)}$ — two reactant species ($ClO$, $O$) collide $\Rightarrow$ bimolecular (molecularity = 2).
2. Reaction intermediate and overall reaction: $ClO$ is produced in step (i) and completely consumed in step (ii) — it does not appear in the initial reactants or final products, so $ClO$ is the reaction intermediate. Adding the two elementary steps: $Cl + O_3 + ClO + O \rightarrow ClO + O_2 + Cl + O_2$ …