Ernest Rutherford found that radioactive atoms release radiation in order to take on a more stable form.
He called the process radioactive decay.
As radioactive elements decay, they change into other elements until they are no longer radioactive, or are stable.
Radioactive decay takes place at different rates in different elements.
Rutherford called an element’s rate of decay its half-life. This is the length of time needed for half the atoms in a sample to decay.
For example, the half-life of radium 226 is about 1,600 years.
After 1,600 years, one half of the radium in a sample remains. After another 1,600 years, only a fourth remains, and so on.
Half-lives of radioactive elements vary from fractions of a second to billions of years.