Radioactive isotope
A radioactive isotope has an unstable nucleus that decays into another isotope at a steady, measurable rate.
A radioactive isotope, or radionuclide, has an unstable nucleus. Over time it decays, giving off particles or energy and turning into a different isotope, often of another element. The original is called the parent and the product the daughter.
The key property is the half-life, the time it takes for half of any sample to decay. Half-lives range from fractions of a second to many billions of years, and they are not affected by temperature, pressure or chemistry. That reliability makes radioactive isotopes natural clocks.
Meteorites contain both long-lived radionuclides, such as uranium-238 and potassium-40, and evidence of short-lived ones that have now died out, such as aluminium-26. The long-lived ones give absolute ages in billions of years; the short-lived ones give precise relative ages for events in the first few million years. The radioactivity in meteorites is extremely weak, far too low to be harmful.
Related terms
Level: Explorer Ages 10 and up