Molecularity

In chemistry, molecularity is the number of colliding molecular entities that are involved in a single reaction step. While the order of a reaction is derived experimentally, the molecularity is a theoretical concept and can only be applied to elementary reaction.The reaction order, the molecularity and the stoichiometric coefficient are the same in elementary reaction although only numerically, because they are different concepts.

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Elementary reactions are classified according to their molecularity. The number of reacting species involved in simultaneous collision to bring about a chemical reaction is called the molecularity of the reaction.