Potassium argon radiometric dating
Symbolically, the process of radioactive decay can be expressed by the following differential equation, where N is the quantity of decaying nuclei and k is a positive number called the exponential decay constant.
The meaning of this equation is that the rate of change of the number of nuclei over time is proportional only to the number of nuclei.
To date older fossils, other methods are used, such as potassium-argon or argon-argon dating.
The time that it takes for half of a sample to decay is known as the half life of the isotope.
Most rocks contain uranium, allowing uranium-lead and similar methods to date them.
Other elements used for dating, such as rubidium, occur in some minerals but not others, restricting usefulness.
The key is to measure an isotope that has had time to decay a measurable amount, but not so much as to only leave a trace remaining.
Given isotopes are useful for dating over a range from a fraction of their half life to about four or five times their half life.
This leaves out important information which would tell you how precise is the dating result.