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   ocr: N THEORY, all radioactive substances will eventually form lead: the nuclei (centers) oflead atoms are stable and are therefore not radioactive. However, this has never been proved, because some half-lives run for longer than humans have existed. The number attached to each common element below is called the atomic mass.It indicates a specific form ofthat element, called an isotopic form. The atoms of each isotopic form have a dilferent mass from those ofall other isotopic forms. COMMON ELEMENTS AND THEIR HALI-LIVES Uranium-238 5b billion years Uranium-235 700 million years Plutonium-239 24,000 ...