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).
|Isotopes||equal proton number||,|
|Isotones||equal neutron number||,|
|Isobars||equal mass number||, ,||see beta decay|
|Mirror nuclei||neutron and proton number exchanged||,|
|Nuclear isomers||different energy states||long-lived or stable|
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 (T1/2 = 4.5×109 a) of uranium occurs in nature, but the shorter-lived isotope, (T1/2 = 0.7 ×109 a), is 138 times rarer. The second group consists of isotopes such as (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 (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)