An atom consists of a small nucleus surrounded by a cloud of electrons. The nucleus is made of protons and neutrons, collectively called nucleons. All atoms of a specific chemical element have the same atomic number (number of protons or electrons) but differing numbers of neutrons. The total number of protons and neutrons in an atom is called its nucleon number. Forms of chemically identical atoms with differing nucleon numbers are called isotopes.

Salem Press Encyclopedia of Science, 2023. 2p.