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A nuclide (from nucleus) is an atomic species characterized by the specific constitution of its nucleus, i.e., by its number of protons Z, its number of neutrons N, and its energy state.[1] Thus, all nuclides are atoms which have at least one electron (though certain ions may be included), but naked nuclei (such as those occurring in cosmic rays and sufficiently hot plasmas) do not technically qualify for the term (these are instead naked nuclei of various nuclear species(elements)). In short, a nuclide is an umbrella term for all nuclei with one or more electrons orbiting.

A set of nuclides with equal proton number (atomic number), i.e. of the same chemical element, but different neutron numbers, are called isotopes of the element. Particular nuclides are still often loosely called "isotopes", but the term "nuclide" is the correct one in general (i.e. when Z is not fixed). Similarly, a set of nuclides with equal mass number A but different atomic number are called isobars (isobar = equal in weight), and isotones are nuclides of equal neutron number but different proton numbers. The name isotone has been derived from the name isotope to remind that in the first group of nuclides it is the number of neutrons (n) that is constant, whereas in the second the number of protons (p).[2]

Nuclear isomers are members of a set of nuclides with equal proton number and equal mass number, but different states of excitation. An example is the two states of 99
shown among the decay schemes. The most long-lived non-ground state nuclear isomer is tantalum-180m, which has a half-life in excess of 1,000 trillion years, and has not been observed to decay to tantalum-180.

There are about 255 nuclides in nature that have never been observed to decay. They occur among the 80 different elements which have one or more stable isotopes. See stable isotope and primordial nuclide. Unstable nuclides are radioactive and are called radionuclides. Their decay products ('daughter' products) are called radiogenic nuclides. About 255 stable and about 84 unstable (radioactive) nuclides exist naturally on Earth, for a total of about 339 naturally occurring nuclides on Earth.

Natural radionuclides may be conveniently subdivided into three types.[citation needed] Firstly, those whose half-lives T1/2 are at least 2% as long as the age of the earth (for practical purposes these are difficult to detect with half lives less than 10% of the age of the Earth) (4.6×109
). These are remnants of nucleosynthesis that occurred in stars before the formation of the solar system. For example, the isotope 238
(T1/2 = 4.5×109
) of uranium is still fairly abundant in nature, but the shorter-lived isotope, 235
(T1/2 = 0.7×109
), is 138 times rarer. About 33 of these nuclides have been discovered (see list of nuclides and primordial nuclide for details).

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