Molecularity of a reaction is best defined as:
AThe number of reacting species (atoms/ions/molecules) that simultaneously collide to bring about an *elementary* reaction
BThe same as the stoichiometric coefficients in a balanced equation
CThe sum of exponents in the rate law
DThe number of products in the reaction
Answer & Solution
Correct answer: A. The number of reacting species (atoms/ions/molecules) that simultaneously collide to bring about an *elementary* reaction
**Molecularity** applies only to **elementary reactions** = number of reactant species colliding simultaneously. **1 = unimolecular**, **2 = bimolecular**, **3 = trimolecular (rare)**. (Order applies to overall rate law; molecularity to each elementary step.)
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