RadioactivityThe natural radioactive decay of atoms has occurred on the earth since its formation, over 4000 million years ago. Radioactivity is the property exhibited by some types of matter to spontaneously emit energy and subatomic particles. It is, in essence, an attribute of individual atomic nuclei. An unstable nucleus will spontaneously decompose, or decay, into a more stable configuration, but it will only do so in some specific ways by emitting certain particles or certain forms of electromagnetic energy. Radioactive decay is a property of numerous naturally occurring elements as well as isotopes of artificially produced elements. The rate at which a radioactive element decays is expressed in terms of its half-life; that is, the time it takes for half of a given amount of isotope to decay. The half-life varies from more than 1,000,000,000 years for some nuclei to less than 10-9 seconds. The product of a radioactive decay process, called a daughter of the parent isotope, may itself be unstable, in which case it too will decay. The process continues until a stable nuclide is formed. The discovery of radioactivity in uranium by the French physicist Antoine Henri Becquerel (1852 - 1908) in 1896 forced scientists to radically change their ideas about atomic structure. Radioactivity demonstrated that the atom was neither indivisible nor immutable. Instead of simply serving as an inert matrix for electrons, the atom could change shape and emit enormous amounts of energy. Furthermore, radioactivity itself became an important tool for revealing the interior of the atom. The German physicist Wilhelm Conrad Röntgen (1845 - 1923) had discovered X-rays in 1895 and Becquerel thought they might be linked to fluorescence and phosphorescence, processes in which substances absorb and emit energy. how light. During his investigations, Becquerel kept some photographic plates and uranium salts in a desk drawer. Expecting to find the plates only slightly cloudy, he developed them and was surprised to find clear images of the salts. He then began experiments that demonstrated that uranium salts emit a penetrating effect