Parent and daughter isotopes
In radioactive decay, the parent isotope is the one that decays and the daughter isotope is the one it turns into.
Radioactive decay is a one-way journey. The isotope that decays is the parent, and the isotope it becomes is the daughter. Uranium-238 is the parent of lead-206, after a chain of intermediate steps; potassium-40 is a parent of argon-40; rubidium-87 is the parent of strontium-87.
Dating relies on measuring both. As time passes, parent atoms decrease and daughter atoms increase at a known rate. If scientists can measure how much of each is present, and know or can work out how much daughter was there at the start, they can calculate how much time has passed.
The complication is that rocks often start with some daughter already present, and later events can add or remove atoms. Methods such as isochrons solve the first problem by comparing several minerals. Choosing parent-daughter pairs that respond differently to heating helps with the second: some clocks are reset easily by impacts, others hardly at all, so together they reveal a rock's whole history.
Related terms
Level: Explorer Ages 10 and up