Nuclide

A nuclide is a type of atom defined by the specific number of protons and neutrons in its nucleus, as well as its nuclear energy state. The term was introduced in 1947 by American physicist Truman P. Kohman, who described it as a "species of atom characterized by the constitution of its nucleus." This definition originally emphasized the nucleus itself.

The concept of a nuclide differs from that of an isotope. While isotopes refer to atoms of the same element (same number of protons) with different neutron counts, the term nuclide was created to focus on nuclear properties apart from chemical behavior. For example, carbon-13 (⁶¹³C) has 6 protons and 7 neutrons. Although "isotope" remains common in fields like nuclear technology and medicine, "nuclide" is the more precise term when discussing nuclear characteristics. In nuclear physics, the number of neutrons can be as important as the number of protons—unlike in chemistry, where neutron count has only a minor effect, except for hydrogen and helium. For hydrogen, the isotope effect is strong enough to influence biological systems. For helium, ⁴₂He follows Bose–Einstein statistics, while ³₂He follows Fermi–Dirac statistics, leading to major differences in low-temperature physical properties.

Though often used interchangeably, "nuclide" and "isotope" describe different relationships. Isotopes are nuclides with the same proton number (same element) but different neutron numbers. Isobars are nuclides with the same mass number but different atomic numbers. Isotones share the same neutron number but differ in proton number. Isodiaphers have the same neutron excess (N − Z). The term "isotone" was modeled after "isotope" to highlight that the constant feature is the neutron count. Nuclear isomers are a special case: they are nuclides with the same proton number and mass number (thus the same isotope) but different excitation states. For instance, technetium-99m and technetium-99 are two different nuclides of the same isotope. The longest-lived nuclear isomer is tantalum-180m, with a half-life exceeding 10¹⁷ years; it occurs naturally and has never been observed to decay to its ground state. In contrast, the ground-state tantalum-180 decays in about 8 hours.

There are 251 nuclides in nature that have never been observed to decay, found among 80 elements with at least one stable isotope. Unstable nuclides are radioactive and called radionuclides; their decay products are radiogenic nuclides.

Naturally occurring radionuclides fall into three groups. First, primordial radionuclides have half-lives long enough to survive from the Earth's formation (about 4.6×10⁹ years ago), originating from stellar nucleosynthesis before the Solar System formed. For example, uranium-238 (half-life 4.463×10⁹ years) is still common, while uranium-235 (half-life 0.704×10⁹ years) is now 138 times rarer. Thirty-five such primordial radionuclides are known. The second group consists of radiogenic nuclides, like radium-226 (half-life 1600 years), which form through radioactive decay within the decay chains of primordial uranium or thorium. Some of these, such as isotopes of francium, are very short-lived; about 50 daughter nuclides exist solely from decay of longer-lived primordial parents. The third group includes nuclides produced by natural nuclear reactions, not simple spontaneous decay. These arise when atoms interact with natural neutrons (from cosmic rays, spontaneous fission, or other sources) or are directly bombarded by cosmic rays. Those from cosmic rays are called cosmogenic nuclides; others from natural nuclear reactions are nucleogenic nuclides. Products of spontaneous fission vary widely but quickly decay—over geologic time—into either primordial nuclides or one of seven long-lived fission products, which are thus found in nature.