Radiometric dating is least useful for dating
Some radioactive parent isotopes decay almost instantaneously into their stable daughter isotopes; others take billions of years.Another line of evidence is based on the present-day abundances of the various isotopes of lead found in the Earth's crust. Three of these isotopes (lead 206, 207, 208) result from radioactive decay of isotopes of thorium and uranium.The fourth, lead 204, is not the result of radioactive decay.The discovery gave scientists a tool for dating rocks that contain radioactive elements.
Many elements have naturally occurring isotopes, varieties of the element that have different numbers of neutrons in the nucleus.
At any moment, the ratio between them is a measure of the time elapsed, as long as the system remains closed.