Radioactive period or half-life:
The number of nuclei in a radioactive sample is halved after a certain characteristic period of time known as the ‘half-life’ or 'period'. The time taken for this division does not depend on the age of the nuclei, or on the size of the sample. After two half-lives, the size of the sample is quartered (not reduced to zero), a third half-life later we are left with an eighth, and so on. This decay rate with respect to time is known as ‘exponential’, and the half-life of a nucleus is one of the most important properties of a radioactive element