The exponents in the rate law for an elementary reaction are equal to the coefficients in the equation.
An elementary reaction is a single step reaction with a single transition state and no intermediates.
The order of an elementary reaction is equal to its molecularity, so the rate of an elementary reaction can be determined by inspection from the molecularity.
There are three types of elementary reactions.
The transition state for a unimolecular process involves only one molecule:
The transition state for a bimolecular process involves only one molecule:
C. Termolecular reactions
Termolecular reactions involve three molecules in the transition state.
They are rare because it is highly unlikely that three molecules will collide simultaneously.
They would be of the type