• Elementary reactions:- Those reaction which takes place in one step only, are called elementary reactions.
• Complex reactions:- Those reaction which takes place in more than one step and this series of steps in which reaction takes place in called Mechanism of the reaction.
• Molecularity of reaction

The number of ions, atoms colliding to each other simultaneously to give product is called molecularity of reaction.

In case of elementary reaction, the molecularity in equal to the sum of the species, present in a balanced chemical equation.

In case of complex reaction, the molecularity is equal to the sum of species present in slowest step of reaction.

e.g.→ O₂F₂ → O₂ + F₂,                Molecularity = 1 so unimolecular

2 HI → H₂ + I₂,                                Molecularity = 2 so Bimolecular

2 NO + O₂ → 2 NO₂,                     Molecularity = 3 so Termolecular

5 Br^- + BrO₃^- + 6 H⁺ → 3 Br₂ + 3 H₂O

Reaction involves 12 ions, impossible that all 12 ions comes together simultaneously to collide. In this rate of reaction depends upon slowest step among many steps of this complex reaction slowest step of reaction is also called the rate determining step.

Characteristics of molecularity of a reaction

• Molecularity is always a whole no. it can never be –ve, zero and in fraction.
• Slowest step decides the rate of reaction or molecularity
• 2^nd step of a reaction is always fast and I^st steps is always slow.
• Rate law expression is always slow step.

Question:- Difference b/w order and molecularity of a reaction.

Ans:- Order of reaction	Molecularity of reaction
1. It is the sum of power to which concentration of reactant must be raised in the rate law	1. It is the no. of ions, atoms or moleculer that must collide to each other simultaneously to give product.
2. Order can be zero, -ve, +ve or in fraction	2. Molecularity is always
3. It is an experimental concept	3. It is an theoretical concept that can be calculated by balanced chemical equation
4. Order may or may not be equal to the No. of molecules in balanced chemical equation	4. It is always equal to no. of molecules from the balanced chemical equation

Mechanism of a reaction

A stepwise description of reaction is called Mechanism of  reaction.

The slowest step is the rate determining step. e.g.→ Rate laws derived from the slowest step.

e.g.→ 2 N₂O₅ → 4 NO₂ + O₂

Rate ∝ [N₂O₅]

Rate = K [N₂O₅] 

Step 1   

Step 2  

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Overall      2 N₂O₅ → 4 NO₂ + O₂

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_{Slowest step is unimolecular so reaction is unimolecular or Ist order.}