Each of these unstable isotopes has its own characteristic half life.
Some half lives are several billion years long, and others are as short as a ten-thousandth of a second.
Some elements have forms (called isotopes) with unstable atomic nuclei that have a tendency to change, or decay.
For example, U-235 is an unstable isotope of uranium that has 92 protons and 143 neutrons in the nucl eus of each atom.
This activity on determining age of rocks and fossils is intended for 8th or 9th grade students.