Elementary Reaction
A reaction that occurs in a single step exactly as written, with no detectable intermediate.
Formula / equation
order = molecularity
Unit
Dimensionless
In depth
For an elementary step alone, the rate law can be written directly from the equation, since order equals molecularity. Real overall reactions are almost always sequences of elementary steps, and identifying that sequence is the goal of mechanistic study.
Examples in the real world
O₃ + NO → O₂ + NO₂ is elementary and therefore second order overall.