Nuclide

A nuclide is a class of atoms characterized by their nuclear properties: atomic number $Z$, neutron number $N$ (or mass number $A$) and nuclear energy state¹. For instance, $\ce{^{12}_{6}C}$ is a stable carbon nuclide.

As a classification scheme, nuclides are essentially the more specific sibling of chemical elements, which only use the atomic number to classify atoms. For instance, $\ce{^{12}_{6}C}$ and $\ce{^{14}_{6}C}$ are the same element (carbon, same atomic number 6), but different nuclides.

It differs from a nucleus in that it is a category representing a type of nucleus through its nucleon configuration, not any particular physical object. In a way, a nuclide is to a nucleus what a chemical element is to an atom.

Stability#

A nuclide is conventionally said to be (observationally) stable if it has never been observed to decay. Numerically, it is stable if its mean lifetime is longer than the age of the Solar System, which is about 4.6 billion years. It is unstable otherwise. Unstable nuclides are also called radionuclides, since they exhibit radioactive decay.

Instability can arise from several factors; the semi-empirical mass formula provides a good estimate of a nucleus’s binding energy, which we can use to model instability. The lower this energy, the less stable the nucleus (except for very light nuclei at the start of the curve, which are exceptions). See Binding energy > Nuclear binding energy for more discussion.

:::image(sidebar)