Each isotope is identified with what is called a ‘mass number’.
When ‘parent’ uranium-238 decays, for example, it produces subatomic particles, energy and ‘daughter’ lead-206.
These use radioactive minerals in rocks as geological clocks.
The atoms of some chemical elements have different forms, called isotopes.
Geologists often need to know the age of material that they find.
They use absolute dating methods, sometimes called numerical dating, to give rocks an actual date, or date range, in number of years.