Radiometric dating parent and daughter
Shortly after Becquerel's find, Marie Curie, a French chemist, isolated another highly radioactive element, .
To measure the passage of long periods of time, scientists take advantage of a regularity in certain unstable atoms.
It has become increasingly clear that these radiometric dating techniques agree with each other and as a whole, present a coherent picture in which the Earth was created a very long time ago.
Further evidence comes from the complete agreement between radiometric dates and other dating methods such as counting tree rings or glacier ice core layers.
One half-life is the time it takes for ½ of the parent isotopes present in a rock or bone or shell to decay to daughter isotopes.
Parent isotopes decay to daughter isotopes at a steady, exponential rate that is constant for each pair.