Explain the Mechanism of a Reaction?

A series of step reactions or elementary processes proposed to account for the overall reaction is called the mechanism of the reaction.

The writing of step reactions is usually based upon the experimental evidence. For example:
The detection of the presence of some short lived intermediates etc. however a complete certainty of the steps is very rarely possible. But one thing is certain I.e., slowest step known as rate determining step must involve the molecules on which the rate of reaction actually depends as observed experimentally and written in the Rate Law equation.

For example:
Combination of NO₂ and F₂ to from NO₂F:
The reaction may be represented as:
2 NO₂ + F₂ ————-> 2 NO₂F
Experimentally, it is found that:
Rate of reaction = k [NO₂] [F₂]
I.e. the rate law equation involves only one molecule of NO₂ and one molecule of F₂. Hence the mechanism will be:
Step 1: NO₂ + F₂ ———-> NO₂F …………….. Slow
Step 2: NO₂ + F ———-> NO₂F …………….. Fast
Hence, the above reaction must be a bimolecular reaction or a reaction of the second order.