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Radioactivity is the phenomenon that the atomic nuclei of some chemical elements may radiate electromagnetic energy and subatomic particles. Nuclei can radiate spontaneously or their radiation can be induced by nuclear fission (splitting) or nuclear fusion (merging of nuclei). Energy release by radioactive nuclei is in the gamma radiation range of the electromagnetic spectrum and as kinetic energy of particles such as neutrons, alpha particles, and beta particles.

Not all chemical elements exhibit radioactivity, specific elements may have stable (i.e., non-radioactive) and radioactive (unstable) isotopes. Nuclei are composed of protons and—with the exception of the hydrogen nucleus—of one or more neutrons. The number of protons defines the atomic number, and the total number of protons and neutrons defines the atomic mass. A radioactive isotope lacks some number of neutrons needed to stabilize the nucleus and in general an unstable nucleus will radiate while decaying to a more stable one.

Quantum mechanics postulates that is impossible to predict at which point in time a given nucleus will emit particles or γ rays, but statistics may be applied to a sufficiently large number of nuclei and it becomes possible to speak of the half-life, which is the time-period that half the nuclei have undergone radioactive decay.

For example, uranium, with an atomic number of 92, has the relatively stable isotope 238U with a half-life of 4.46 × 109 years. Although this is a very long period, a sufficiently large collection of   238U nuclei emits detectable amounts of α particles and even its decay product, the 234 isotope of thorium, emits detectable amounts of β particles.

Radioactivity was first discovered in uranium salts. Henri Becquerel noted on the first of March 1896 that the uranyl salt K2UO2(SO2)2•(H2O)2 blackens photographic film after several days of contact. It is now believed that the blackening observed by Becquerel was caused by the β radiation from uranium's "daughter" thorium. In 1898 Marie Curie and her husband Pierre Curie coined the name "radioactive substance" and afterwards it was a small step to the term radioactivity.

Uranium's 235 isotope, which occurs in nature with an abundance of about 0.7%, is only slightly less stable than the 238 isotope, with a half-life of 7.13 × 108 years. It will fission into two almost equal parts when it is struck by a neutron and then it releases enormous amounts of energy, which makes the 235 isotope suitable as fuel for nuclear reactors and as explosive in nuclear weapons.

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