Rubidium strontium dating example
It was discovered that some chemical elements, notably uranium and thorium, are strongly radioactive.
These elements occur naturally in nearly all rocks, and they account for the radioactivity you could observe with a Geiger counter.
Unfortunately, there is a lot of primordial Strontium 87 around, making it very hard to tell how much was present when the rock formed and how much formed later as a result of radioactive decay.
Fortunately, there is another isotope of strontium around, Strontium 86. Furthermore, since the two isotopes differ by only about 1% in weight, they do not undergo appreciable fractionation from physical processes.
The radioactive decay process can be described simply as the transformation of an unstable radioactive atom (called the parent) to a new atom (called the daughter) that may differ in atomic number, atomic mass, or both.
The number of protons in the nucleus of an atom is called its atomic number.
The above equation tells us the absolute amount of Strontium-87 in the rock sample.