01Single-step events
An elementary step describes one actual collision. Only for elementary steps can you write the rate law straight from coefficients.
Molecularity: unimolecular (1 particle), bimolecular (2). Termolecular steps are rare — three-way collisions are unlikely.
02Notebook box
Elementary 2NO → N₂O₂: rate = k[NO]².
Worked example: A + B → C elementary → rate = k[A][B].