The methods work because radioactive elements are unstable, and they are always trying to move to a more stable state. This process by which an unstable atomic nucleus loses energy by releasing radiation is called radioactive decay.
The thing that makes this decay process so valuable for determining the age of an object is that each radioactive isotope decays at its own fixed rate, which is expressed in terms of its half-life.
So, if you know the radioactive isotope found in a substance and the isotope's half-life, you can calculate the age of the substance. Well, a simple explanation is that it is the time required for a quantity to fall to half of its starting value.
So, you might say that the 'full-life' of a radioactive isotope ends when it has given off all of its radiation and reaches a point of being non-radioactive.
Compared to some of the other radioactive isotopes we have discussed, carbon-14's half-life of 5,730 years is considerably shorter, as it decays into nitrogen-14.For example, how do we know that the Iceman, whose frozen body was chipped out of glacial ice in 1991, is 5,300 years old?Well, we know this because samples of his bones and hair and even his grass boots and leather belongings were subjected to radiocarbon dating.With rubidium-strontium dating, we see that rubidium-87 decays into strontium-87 with a half-life of 50 billion years.By anyone's standards, 50 billion years is a long time.