https://doi.org/10.1351/goldbook.L03515
Lifetime of a molecular entity, which decays by first-order kinetics, is the time needed for a concentration of the entity to decrease to of its original value, i.e., . Statistically, it represents the life expectation of the entity. It is equal to the reciprocal of the sum of the first-order rate constants of all processes causing the decay of the molecular entity.
Notes:
- Mathematical definition:
with the first-order rate constants for all decay processes of the decaying state. - Lifetime is used sometimes for processes, which are not first order. However, in such cases, the lifetime depends on the initial concentration of the entity, or of a quencherand, therefore, only an initial or a mean lifetime can be defined. In this case it should be called decay time.
- Occasionally, the term half-life (
) is used, representing the time needed for the concentration of an entity to decrease to one half of its original value, i.e., . For first-order reactions, .