The atomic nucleus is the central region of an atom, composed of a collection of protons and neutrons (collectively termed nucleons) bound together by the strong interaction. The nucleus contains almost all the atom’s mass and carries a net positive electric charge. Despite the heavy mass, its size is tiny compared to that the whole atom: nuclear radii are usually in the order of , as opposed to atomic sizes which are .
The existence of a dense central nucleus was discovered by Rutherford in 1911 through the Rutherford experiment.
A nuclide is a related concept that classifies atoms based on their nucleus.
Stability#
Atomic nuclei can be unstable, which means that they undergo spontaneous nuclear decay, a process that generally leads to the expulsion of one or more nucleons in order to remove sources of instability. Instability becomes progressively more common as nuclear binding energy per nucleon reduces, which corresponds to increasing . Moreover, instability is caused by excesses in number of protons or neutrons. There are three main categories of decay: alpha decay, beta decay and gamma decay. See Nuclide > Stability for more.