Radiometric dating is a means of determining the "age" of a mineral specimen by determining the relative amounts present of certain radioactive elements.
Rubidium-Strontium dating: The nuclide rubidium-87 decays, with a half life of 48.8 billion years, to strontium-87.
Strontium-87 is a stable element; it does not undergo further radioactive decay.
It has the same number of protons, otherwise it wouldn't be uranium.
The number of protons in the nucleus of an atom is called its atomic number.
If we knew the fraction of a radioactive element still remaining in a mineral, it would be a simple matter to calculate its age by the formula To determine the fraction still remaining, we must know both the amount now present and also the amount present when the mineral was formed.