A nuclide is an atomic nucleus with a specified composition of protons and neutrons.

The nuclide concept emphasizes nuclear properties over chemical properties while

the isotopeconcept does the converse; for the neutron number has drastic effects on nuclear

properties but negligible effects on chemical properties. Since isotope is the older term, it is

better known, and it is still sometimes used in contexts where nuclide would be more proper, such as nuclear technology.

An isotope and/or nuclide is specified by the name of the particular element (this indicates the

atomic number implicitly) followed by a hyphen and the mass number (e.g. helium-3,carbon-

12, carbon-13, iodine-131 and uranium-238). When a chemical symbol is used, e.g., "C" for

carbon, standard notation is to indicate the number of nucleons with a superscript at the

upper left of the chemical symbol and to indicate the atomic number with a subscript at the

lower left

(e.g. 32He, 42He, 126C, 146C, 23592U, and 23992U).

Some isotopes are radioactive and are therefore described as radioisotopes or radionuclides

, while others have never been observed to undergo radioactive decay and are described

asstable isotopes. For example, ¹⁴C is a radioactive form of carbon while ¹²C and ¹³C are

stable isotopes. There are about 339 naturally occurring nuclides on Earth^[2], of which 288

areprimordial nuclides and 259 are "stable"^[2]. However, some apparently "stable" isotopes

are predicted by theory to be radioactive with very long half-lives.^{[citation needed]} Adding in

the radioactive nuclides that have been created artificially, there are more than 3100 currently

known nuclides.^[3]