in chemistry, the minimum amount of energy that is required to activate atoms or molecules to a condition in which they can undergo chemical transformation or physical transport. In terms of the transition-state theory (q.v.), the activation energy is the difference in energy content between atoms or molecules in an activated or transition-state configuration and the corresponding atoms and molecules in their initial configuration. The activation energy is usually represented by the symbol E_a in mathematical expressions for such quantities as the reaction-rate constant, k = Aexp(-E_a/RT), and the diffusion coefficient, D = D_oexp(-E_a/RT).

Activation energies are determined from experimental rate constants or diffusion coefficients that are measured at different temperatures.