Constituents of a Nucleus

The atomic nucleus is built from two types of subatomic particles, together called nucleons: protons and neutrons. A proton carries a positive electric charge, equal in magnitude to the (negative) charge of an electron, and has a mass about 1836 times that of an electron. A neutron, as its name suggests, carries no electric charge at all, and has a mass very slightly greater than that of a proton (see the mass table for exact values in several convenient units).

The number of protons in a nucleus is called the atomic number, denoted Z; since a neutral atom has exactly as many electrons as protons, Z also equals the number of electrons, and it is Z alone -- not the number of neutrons -- that determines an element's chemical identity and chemical properties. The number of neutrons is denoted N, and the TOTAL number of nucleons (protons plus neutrons together), A = Z + N, is called the mass number, since it very closely determines the total mass of the nucleus (and hence of the atom). An atom of element X is conventionally written $_Z^AX$; for example, ordinary hydrogen, carbon and oxygen are written $_1^1H$, $_6^{12}C$ and $_8^{16}O$. …