(of atom) The central core of an atom that contains most of its mass. Experiments performed in 1909 by Geiger and Marsden on the scattering of alpha particles (under the direction of Rutherford) led to the discovery of a nuclear structure in 1911 (see Rutherford scattering). The nucleus is positively charged and contains one or more nucleons (protons or neutrons). The positive charge of the nucleus is determined by the number of protons it contains (see atomic number); in the neutral atom this positive charge is balanced by an equal number of negatively charged electrons orbiting the nucleus in a comparatively large region outside it.

The simplest nucleus is the hydrogen nucleus, consisting of a single proton. All other nuclei contain neutrons, which contribute to the atomic mass (see nucleon number) but not to the nuclear charge. Therefore nuclei with different numbers of neutrons but the same number of protons will possess the same number of electrons around the nuclei of their neutral atoms. Such atoms are called isotopes. Since it is the electron configuration around the nucleus that dictates chemical properties, isotopes are chemically indistinguishable and separation is achieved by nonchemical methods (see isotope separation). Nuclei do not seem to have clearly defined perimeters. However, an estimate of a nuclear radius may be achieved by investigating the scattering of very high-energy alpha particles by the nucleus. The deflections of alpha particles in Rutherford scattering experiments agree very well with predicted data, except when the alpha particles have very high kinetic energies. The theoretical predictions made by Rutherford assumed the nucleus to be a point of positive charge. The incident alpha particle would behave as though the nucleus were a point as long as it did not begin to penetrate the outer regions of the nucleus itself. Deviation of the alpha particle scattering data from the point nucleus predictions, indicates the onset of nucleus penetration. Experiments of this kind, using a wide variety of target nuclei, reveal that the radius R of any nucleus can be represented by: R=r₀A^⅓, where r₀ is a constant (=1.414×10⁻¹⁵ m) and A is the nucleon number of the nucleus. Nuclear radii are commonly expressed in femtometres (1 fm=10⁻¹⁵ m), which is sometimes called a fermi. Experiments using projectiles other than alpha particles (such as electrons, neutrons, etc.) give slightly different values of r₀.

The relationship between R and A has an interesting implication for the density of nuclear material. Since both mass and volume of nuclear material are proportional to A, an expression for the density of nuclear material is constant and therefore the same for all nuclei; that is, all nuclei have the same density, estimated to be about 2.3×10¹⁷ kg/m³. See also liquid-drop model.