Heterogeneous catalysis relies to a large extent on the reactivity of metal nanoparticles. The surface of these particles consists of atomically smooth terraces and edges. As local environments of atoms in edges and terraces are different, their catalytic ability varies. This severely complicates accurate predictions of reaction kinetics in heterogeneous catalysis. In this study, we use the reaction dynamics of H₂ dissociation on a series of [n(111) × (100)] stepped platinum single-crystal surfaces to resolve how reactivity for atoms in edges and terraces can be separated and quantified individually. A simple reactivity model that only requires input from n = 3 accurately predicts reactivity for any combination of a (100) step with a (111) terrace over the entire energy range of interest for incident molecules. Our results support the assumption in theoretical kinetics studies that the smallest unit cell accurately models the essential features of the catalytic surface, and we discuss limitations to this assumption. Finally, from our model, we quantify for the first time the absolute reaction cross section for a direct dissociative process in a gas–surface collision.