Answer with explanation:
Radioactive elements or isotopes are atoms which release radiations over time and this cause decrease in their amount and convert to other isotope. This is also known as radioactive decay.
Half life: Half life of an radioactive element is the time, after which the amount of element remains half.
To accurately date fossils, we should know the half life of an element.
In radioactive decay one isotope change to other by emitting radiations with time. We check the amount of secondary isotope and then relate it to half life of element to know the time that how much time take to make this secondary element.