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Radioisotopic dating is a key tool for studying the timing of both Earth's and life's history.
This suite of techniques allows scientists to figure out the dates that ancient rock strata were laid down and hence, provides information about geologic processes, as well as evolutionary processes that acted upon the organisms preserved as fossils in interleaved strata.
For example, over time, uranium atoms lose alpha particles (each made up of two protons and two neutrons) and decay, via a chain of unstable daughters, into stable lead.
Although it is impossible to predict when a particular unstable atom will decay, the decay rate is predictable for a very large number of atoms.
Uranium comes in two common isotopes with atomic weights of 235 and 238 (we'll call them 235U and 238U).
When an unstable Uranium (U) isotope decays, it turns into an isotope of the element Lead (Pb).
Of all the isotopic dating methods in use today, the uranium-lead method is the oldest and, when done carefully, the most reliable.
Unlike any other method, uranium-lead has a natural cross-check built into it that shows when nature has tampered with the evidence.
Uranium 235 decays to lead 207, and Thorium 232 decays to lead 208.
In addition there is another stable isotope, lead 204, that is entirely primordial and does not form via radioactive decay at all.