It has already been pointed out that each radioactive isotope has a specific half-life period.
The radiations that it emits can be detected and measured.
Also the chemical properties of isotopes of a given element are identical.
This makes it possible to incorporate a small amount of radioisotopes in a system and trace the fate of particular element or a compound in a series of chemical or physical changes.
We assume that the rock did not contain any lead isotope initially.Suppose, the ratio of uranium-238 and lead-206 is unity.It implies that half of the uranium originally present has been converted into lead isotope.The age of rock must therefore be equal to the half-life of uranium-238 i.e., 4.5 x 109 years.Pb/U ratio of most of the rocks is 1.33 x 10-2 indicating that their age is of the order of 10s years. The abundance ratio of the two isotopes of uranium, i.e., 235U : 238U at present is 1 : 140. Assuming that in the beginning the proportion of two isotopes was equal, the above data determines the age of earth as 5 x 109 years.RADIO CARBON DATING It is a method of determining the ages of the archeological objects (wood, dead plants and animals). It helps in determining the date at which a particular plant or animal died. The principle of this technique lies in the fact that due to bombardment of cosmic rays, nitrogen atoms present in the upper atmosphere are converted into radioactive carbon, 14C according to the reaction: Carbon-14 is radioactive and has half-life period of 5770 years.