According to Collision theory a chemical reaction takes place due to collision between the particles of these reactants. The number of reacting species (atoms, ions or molecules) which must collide simultaneously in order to bring about the chemical reaction called molecularity of the reaction. The molecularity of the reaction. The molecularity of the reaction can be 1, 2, or 3. For example:
    
The decomposition of ammonium nitrate is a unimolecular reaction.
                                        
NH₄NO₂    N₂ + 2H₂O
    
The reaction involving simultaneous collision between two species is a bimolecular reaction. For example, dissociation of HI is bimolecular reaction.
                                     
2HI(g)    H₂(g) + I₂(g)
    
In the same way, the reaction between NO and O₂ is a trimolecular reaction because it involve collisions among three species
                                      
2NO + O₂    2NO₂

In all the above chemical reaction molecularity is simply the sum of molecules of the different reactants as represented by the balanced chemical equation. Such reactions are known as elementary reactions.

For most of the reactions, the molecular dues not exceed three. It is for the reason that the chances of simultaneous collisions among three or more particles are rare.

A perusal of large number of chemical reactions shows that the balanced equation may contain four or more species/molecules/ions participating in the reaction. Some widespread examples are:
    
N₂ + 3H₂    2NH₃
    
4BHr + O₂    2H₂O + 2Br₃
    
2MnO₄^-1 + 5C₂O₄^2- + 16H⁺    2Mn⁺² + 10CO₂ + 8H₂O

On the basis of balanced equations the molecularity of above reactions is 4, 5 and 23 respectively.

Since, molecularity greater than there is not possible; therefore, the above reaction does not involve the simultaneous collision of all the reacting species in a single step. Actually, such chemical reactions go on through a series of steps. Each step is a basic step and involves the synchronized collision of two or three species only. Such chemical reactions which proceed through more than one step are termed as complex reactions.

The detailed description of various steps of the complex chemical is called mechanism of the reaction. For example, the reaction, 4HBr + O₂    2H₂O + 2Br₂ occurs by the following steps:


                
The different steps of the given reaction are written based upon the experimental evidence like detection of the presence of some short lived intermediation. All the above steps are known to proceed at different rates.

On the basis of the above discussion it is clear that the term molecularity of overall reaction has no significance in this case. As the different steps proceed at different rates a curious question arises that which amongst these equations can be employed to express the rates of such reaction.