= Elementary reaction
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An <elementary reaction> is one microscopic step in a <reaction mechanism>, with no intervening <reaction intermediate> needed within that step. Its <molecularity> counts the participating reacting entities. In an ideal dilute, uniformly mixed model, the <law of mass action> gives the directional <reaction rate> from the step's <reactant> <concentrations>: $\mathrm A\longrightarrow\mathrm P$ has $r=k[\mathrm A]$, while $\mathrm A+\mathrm B\longrightarrow\mathrm P$ has $r=k[\mathrm A][\mathrm B]$. Applying those powers to a net <chemical equation> requires separate evidence that it is elementary.<Chemistry 2e by Paul Flowers and others/12.6 Reaction Mechanisms>[Chemistry 2e by Paul Flowers and others; §12.6, Unimolecular Elementary Reactions and Bimolecular Elementary Reactions: step equations and directional rate laws]{ref}
For a reversible step $\mathrm A+\mathrm B\rightleftharpoons\mathrm I$, apply that rule separately: $r_f=k_f[\mathrm A][\mathrm B]$ and $r_r=k_r[\mathrm I]$, giving net production of I as $r_f-r_r$. Their difference can vanish at <chemical equilibrium> while both directional <reaction rates> remain nonzero. For $2\mathrm A\longrightarrow\mathrm P$, the elementary normalized <reaction rate> is $r=k[\mathrm A]^2$, whereas A's <disappearance rate> is $2r$; see <second-order reaction> for the complete factor-of-two example.
Calling a step slow does not remove the need for its actual <reactant> <concentrations>. If a <reaction intermediate> enters its <rate law>, determine that <concentration> from the other <rate equations> or a justified approximation. In the <consecutive first-order reaction mechanism>, an intermediate maximum is only one instant at which its production and consumption match. Replacing its entire evolution by that one balance would lose the initial buildup and delayed <reaction product> formation. A successful overall <first-order reaction> fit likewise does not establish an elementary event: both worked <reaction mechanisms> above provide ways for microscopic detail to remain hidden in one <reactant> trace.
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