Radioisotopic dating method currently used
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.
The formation of crystals in the magma marks the moment that the radio-isotopic clock starts ticking.For practice, use the graph above to estimate the age of a rock sample that contains 10% uranium and 90% lead.The example above describes uranium/lead decay, which happens very slowly; however, different radioactive elements have different half-lives. This allows scientists to date events that are more or less ancient.Now imagine that you have a rock sample that contains 39% uranium-235 and 61% lead-207. At around 1000 million years (i.e., one billion years), as shown on the graph at right above.Thus, you would calculate that your rock is about a billion years old.