Number of neutrons can vary in an element, but number of protons is constant. Is not the same as an element's atomic weight , which is the weighted mean of the masses of an element's constituent isotopes. Have the same number of protons, but different number of neutrons. Under natural conditions, elements occur as mixtures of isotopes. Different isotopes of the same element react chemically in same way.
Some isotopes are radioactive.
Has a fixed half life . Dating geological strata and fossils. Radioactive decay is at a fixed rate. By comparing the ratio of radioactive and stable isotopes in a fossil with the ratio of isotopes in living organisms, one can estimate the age of a fossil. The ratio of 14 C to 12 C is frequently used to date fossils less than 50,000 years old.