S-Cool Revision Summary

S-Cool Revision Summary

Activity is defined as the rate of decay (or number of disintegrations per second) of a substance.

Half life is the time taken for half of the radioactive nuclei present to decay or the time taken for the activity of a sample to halve.

Carbon-14 Dating is a useful example of the concept of half-life in practice. Carbon-14 is a radioactive isotope of carbon with a half-life of 5730 years.

Decay constant is the fraction of the total number of nuclei present that will decay in a unit time.


A = λN

λ = 0.693/ t ½

N = Noexp(-λt)

A = Aoexp(-λt)


A = activity of a sample. (Units: Becquerel, Bq.) Activity depends on number of particles present initially.

Ao = activity at the start

N = number of atoms

No = the number of atoms at the start

λ = the decay constant

t ½ = half life

t = time