Nuclide

A nuclide (from lat.: nucleus) is a nuclear species which is characterized by the number of protons and neutrons that every atomic nucleus of this species contains. It can be used to distinguish isotopes among nuclei, as well as other properties listed below.

For a short-hand designation of the nuclide, one writes the mass number (number of nucleons) in the upper left corner and the atomic number (number of protons) in the lower left corner of the chemical symbol: for example, for the most common isotope of carbon. In earlier years, the mass number was written in the upper right corner. The atomic number may also be omitted, since it is uniquely defined by the element symbol: for example, Carbon. Where needed, the number of neutrons can be written in the lower left corner for clarity: for example,. This is normally omitted, as it can be deduced by subtracting the element number from the mass number.

The various nuclides of a particular chemical element with equal proton number (atomic number), but different neutron numbers are called isotopes of this element. Before the term "nuclide" was internationally accepted (ca. 1950), the term "isotope" was also loosely used to describe a nuclear species, i.e., a nuclide. Nuclides with equal mass number but different atomic number are called isobars (isobar = equal in weight). Isotones are nuclides of equal neutron number but different proton numbers.

Nuclear isomers are atomic nuclei of a particular nuclide that have equal proton number and equal mass number, differ in energy content, and are long-lived (for example the two states of shown among the decay schemes).

Unstable nuclides are radioactive and are called radionuclides. Their decay products ('daughter' products) are called radiogenic nuclides.

About 270 stable and about 70 unstable (radioactive) nuclei exist in nature. There are three main types of natural radionuclides. Firstly, those whose half-lives T1/2 are at least 10% as long as the age of the earth (4.6×109 years). These are remnants of nucleosynthesis that occurred in stars before the formation of the solar system. For example, the isotope 238Uranium (T1/2 = 4.5×109 a) of uranium occurs in nature, but the shorter-lived isotope, 235Uranium (T1/2 = 0.7 ×109 a), is 138 times rarer. The second group consists of isotopes such as 226Radium (T1/2 = 1602 a), an isotope of radium, which are formed in the radioactive decay chains of uranium or thorium. The third group consists of nuclides such as 14Carbon (radiocarbon) that are made by cosmic-ray bombardment of other elements. Many more than 1000 nuclides have been artificially produced.

The known nuclides are shown in charts of the nuclides (see Weblinks)