<Dl> <Dd> N (t) = N 0 (1 2) t t 1 / 2 N (t) = N 0 e − t τ N (t) = N 0 e − λ t (\ displaystyle (\ begin (aligned) N (t) & = N_ (0) \ left ((\ frac (1) (2)) \ right) ^ (\ frac (t) (t_ (1 / 2))) \ \ N (t) & = N_ (0) e ^ (- (\ frac (t) (\ tau))) \ \ N (t) & = N_ (0) e ^ (- \ lambda t) \ end (aligned))) </Dd> </Dl> <Dd> N (t) = N 0 (1 2) t t 1 / 2 N (t) = N 0 e − t τ N (t) = N 0 e − λ t (\ displaystyle (\ begin (aligned) N (t) & = N_ (0) \ left ((\ frac (1) (2)) \ right) ^ (\ frac (t) (t_ (1 / 2))) \ \ N (t) & = N_ (0) e ^ (- (\ frac (t) (\ tau))) \ \ N (t) & = N_ (0) e ^ (- \ lambda t) \ end (aligned))) </Dd> <Dl> <Dd> <Ul> <Li> N is the initial quantity of the substance that will decay (this quantity may be measured in grams, moles, number of atoms, etc .), </Li> <Li> N (t) is the quantity that still remains and has not yet decayed after a time t, </Li> <Li> t is the half - life of the decaying quantity, </Li> <Li> τ is a positive number called the mean lifetime of the decaying quantity, </Li> <Li> λ is a positive number called the decay constant of the decaying quantity . </Li> </Ul> </Dd> </Dl> <Dd> <Ul> <Li> N is the initial quantity of the substance that will decay (this quantity may be measured in grams, moles, number of atoms, etc .), </Li> <Li> N (t) is the quantity that still remains and has not yet decayed after a time t, </Li> <Li> t is the half - life of the decaying quantity, </Li> <Li> τ is a positive number called the mean lifetime of the decaying quantity, </Li> <Li> λ is a positive number called the decay constant of the decaying quantity . </Li> </Ul> </Dd>

Formula for calculating half life of a radioactive substance