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Atomic nuclei of a radioactive element decay spontaneously, producing other elements and isotopes until a stable species is formed. The life span of a single atom may have any value, but a statistical quantity, the half-life of a macroscopic sample, can be measured; this is the time in which one-half of the sample disintegrates. The age of a rock, for example, can be determined by measuring ratios of the parent element and its decay products. The decay of uranium to lead was first used to measure long intervals, but the decays of potassium to argon and of rubidium to strontium ...(100 of 14824 words)