Molecularity

Molecularity is the number of reactant species that collide simultaneously in a single elementary step, so it must be a whole number such as one, two, or three and can never be zero or fractional. Order, in contrast, is an experimentally determined quantity for the overall reaction and can be zero, fractional, or even negative when the reaction proceeds through several steps with a rate-determining step that does not reflect simple stoichiometry. Therefore order can be fractional while molecularity cannot. The option that molecularity can be fractional is wrong because molecularity refers to discrete colliding particles. The option that both can be fractional incorrectly extends fractional values to molecularity. The option that neither can be fractional ignores well-documented fractional-order reactions. This distinction between an empirical macroscopic quantity and a microscopic mechanistic count is emphasised in NCERT. It is worth emphasising that this is not a special case but a representative example of how molecularity operates throughout chemical kinetics. Working through the logic step by step, rather than memorising the result, makes it clear why elementary step governs the behaviour seen here. Plausibility check: reactions such as the decomposition of acetaldehyde show order 1.5, confirming fractional order, while no single step can involve a fractional number of molecules.