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Sometimes referred to as a "nuclear species", a nuclide is a species of atom that has a particular number and arrangement of protons and neutrons in it's nucleus. A nuclide is also characterized by it's mass, atomic number (also called "Z", the number of protons), mass defect, and nuclear binding energy.

Another way to put it: Physical scientists use the word 'nuclide' to refer to varieties of the nucleus for a given chemical element’s atoms, each element consisting of a unique species of atoms distinguished by a unique number of nuclear protons, their so-called atomic number Z, each species of atoms, i.e., each element, having a number of nuclides distinguished by the number of neutrons N accompanying the Z protons. A nuclide thus refers to a uniquely identifiable atomic nucleus, each element having one or more such uniquely identifiable atomic nuclei, or nuclides.

If a given chemical element has two or more nuclides, each with a different number of neutrons accompanying the element's unique number of protons, the nuclides characterize the different isotopes of the element, and the nuclides themselves are considered isotopic. For that reason the term nuclide is often used interchangeably with isotope in relation to a given chemical element.

Nuclides can be stable or unstable, though no stable nuclides exist for chemical elements with Z greater than 83. Interposed within the set of chemical elements that do have stable nuclides (Z=1 to Z=83), technetium (Z=43) and promethium (Z=61) have no stable nuclides.

When unstable, one type of nuclide spontaneously transforms into another type according to a set statistical formula. This process emits radiation. The rate at which a radioactive nuclide decays is indicated by it's half-life.

Nuclides may decay by different types of nuclear radiation. Alpha decay occurs when the nucleus emits two protons and two neutrons (a 4Helium nucleus). The 238Uranium nuclide decays by alpha particle emission according to the scheme: 238U--->234Th + 4He where the energy released in the transformation is Q=4.25MeV.

Other types of nuclear emissions can be electrons and positrons, which are grouped together as beta decay, and high energy photon emission called gamma decay.