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The eventual decay of any particular radioactive atom is random – we can not know when it will decay. The half life of an isotope is the time that it takes for half of the atoms to decay.
For example uranium-235 has a half live of around 700,000 years. This means that there is a 50/50 chance that a particular atom will decay in the next 700,000 years. Half of the uranium-235 atoms would be expected to decay in that time.
Every radioisotope has a unique half-life. Some are as short as a nanosecond, some are measured in billions of years. After each half-life has passed, the amount of the radioisotope present has halved.
Radioactive decay is measured using the Becquerel. One Bq = One decay / second.
The rate of decay is dependent on the amount of the radioisotope: more atoms means more chance of decays occurring. Isotopes with shorter half life will decay more quickly – at a higher rate of decay. Long lived isotopes will release smaller amounts of radiation over a longer time period.
Use this interactive to determine the half-life of the isotopes & to find a mathematical relationship to model the decay over time.