Most of the radioactive isotopes used for radioactive dating of rock samples have too many neutrons in the nucleus to be stable.Recall that an isotope is a particular form of an element.Different isotopes of a given element will have the same chemistry but behave differently in Radioactive isotopes will decay in a regular exponential way such that one-half of a given amount of parent material will decay to form daughter material in a time period called a half-life. When the material is liquid or gaseous, the parent and daughter isotopes can escape, but when the material solidifies, they cannot so the ratio of parent to daughter isotopes is frozen in.The parent isotope can only decay, increasing the amount of daughter isotopes. The number n is the number of half-lives the sample has been decaying.
When looking at a large number of atoms, you see that a certain fraction of them will change or dating system because you can determine accurate ages from the number of remaining radioactive atoms in a rock sample.
The number of parent isotopes decreases while the number of daughter isotopes increases but the total of the two added together is a constant.