We now begin our study of the nucleus. A nucleus is a bound assembly of neutrons and protons. AZ x denotes a nucleus of an atom of the chemical element X containing A nucleons, of which Z are protons and N = (A — Z) are neutrons. For example, 35 17Cl denotes a chlorine nucleus with 18 neutrons and 37 17Cl a chlorine nucleus with 20 neutrons. Since the chemical symbol determines the atomic number Z, 35Cl or 37Cl is identification enough, but the addition of the Z label is often useful. A = (N + Z) is called the mass number of the nucleus. Nuclei which differ only in the number of neutrons they contain are called isotopes. Nuclei of the same A but different Z are called isobars.

4.1 Electron scattering by the nuclear charge distribution

Rutherford's famous analysis in 1911 of the scattering of a-particles by matter established that the size of the nucleus of an atom is small compared with the size of the atom. Whereas the electronic distribution extends to a distance of the order of angstroms (1 A = 10-10 m) from the nucleus, these and later experiments showed that the distribution of nucleons is confined to a few fermis (1 fm = 10-15 m). Early theories of α- decay and nuclear binding energies gave estimated values for nuclear radii of a similar magnitude.

Precise information came in the 1950s, with experiments using the elastic scattering of high-energy electrons to probe the nuclear charge distribution.