An Etymological Dictionary of Astronomy and Astrophysics

The central part of the → atom. It is made up of → protons and, in most cases, → neutrons. The nucleus is surrounded by a swarm of fast-moving → electrons. Almost all of the mass (more than 99%) of an atom is contained in the dense nucleus. The number of protons in the nucleus (called → atomic number) determines the type of → chemical element.
Atoms that differ only in the number of neutrons in their nuclei are called → isotopes.

See also: → atomic; → nucleus.