An isotope that undergoes radioactive decay, its nuclei disintegrating spontaneously to form a daughter isotope (often of a different element). For example, rubidium-87 is the parent isotope of strontium-87, into which it decays with a half-life of 4.88 × 10¹⁰ years. The amount of the parent isotope in a sample decreases with time, while that of any daughter isotope (and its own decay products) increases; the relative proportions of the two can be used to derive the age of the sample (see radioactive age dating).