Elementary Reactions and Mechanism:
Elementary reactions are steps of molecular events showing how reactions proceed. This type of description is a mechanism.

The mechanism for the reaction between CO and NO2 is proposed to be

Step 1   NO2 + NO2 –>   NO3 + NO   (an elementary reaction)
Step 2   NO3 + CO –>   NO2 + CO2   (an elementary reaction)

Add these two equations led to the overall reaction

NO2 + CO = NO + CO2     (overall reaction)

Molecularity of Elementary Reactions:
The total order of rate law in an elementary reaction is molecularity.
The rate law of elementary reaction is derived from the equation. The order is the number of reacting molecules because they must collide to react.

1. A molecule decomposes by itself is a unimolecular reaction (step);
2. Two molecules collide and react is a bimolecular reaction (step); &
3. Three molecules collide and react is a termolecular reaction (step).

O3 –>  O2 + O     rate = k [O3]

NO2 + NO2 –>   NO3 + NO    rate = k [NO2]2

Br + Br + Ar –>   Br2 + Ar*    rate = k [Br]2[Ar]

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