NUCLEUS. Modern physical research has led to the theory that an atom of any given element, far from being the minute, impenetrable solid entity which it was supposed to be before the discovery of the electron, is a very open structure. It consists of a small, relatively compact particle at the centre, carrying a net positive charge, which is surrounded by a number of electrons sufficient to make the atom, as a whole, neutral. This central particle, in which practically the whole mass of the atom is concentrated, is exceedingly small, even compared to the atom. It is about a million-millionth of a centimetre in diameter, while the atom itself is a few hundred millionths of a centimetre across. These figures must not be taken to do more than give a rough estimate of the kind of sizes in question, for not only is it difficult to define precisely the size of a particle without a sharp boundary, such as an atom or its parts, but also the size is known to vary somewhat from atom to atom. The minute central particle is called the nucleus of the atom, the term having been first suggested by Rutherford in 1912 (Philosophical Magazine, xxiv, , 461), to whom the nuclear theory owes its origin.
The nuclear theory of the atom is discussed under ATOM, where, however, attention is directed mainly to the widely-spaced elec trons which surround the nucleus, the nucleus itself being con sidered merely as supplying a central attractive force which gov erns the number and behaviour of these electrons. In this article the properties of the nucleus itself will be treated in more detail.
Nuclear Properties.—The nucleus being at the centre of the atom, and surrounded by a strong electric field, is inaccessible to ordinary laboratory agencies, such as heat, strong electric or magnetic fields, mechanical pressure, and so on. Chemical com bination affects only the outer regions of the atom, and has no influence on the nucleus itself. Nuclear properties are therefore atomic properties, rather than molecular properties : that is, they persist unchanged as properties of the atom, no matter how it may be combined. Such a property is mass, which has been experi mentally shown, to a very high degree of accuracy, to be un changed by chemical reactions. Another property of the nucleus is its net positive charge, which determines how many electrons an atom can possess in its neutral state, and so determines its chemical properties, since a given number of electrons controlled by a given central charge take up a definite disposition. To these mass and charge properties of the nucleus we attribute the exist ence of isotopes (q.v.), since, as we shall see, a given nuclear charge is not invariably associated with the same nuclear mass. The radioactive properties, which no laboratory agent can pro duce or modify, hurry up, or slow down, are also of an atomic nature. There is strong evidence that the radioactive processes which give rise to the discharge of a-, 0-, and 7-rays arise in the nucleus : radioactivity is a nuclear process. It is, in fact, mainly
from the radioactive side that our knowledge of nuclear structure, scanty though it may be, has come.
The experiments, mainly carried out by Aston, which have established firmly the existence of isotopes, reveal the net charge and mass of the nucleus, but tell us little of any mechanism which may exist within the nucleus. We may therefore say that the study of isotopes reveals the static properties of the nucleus. On the other hand radioactive processes arise in energy changes which are taking place within the nucleus. Their study reveals the dynamical aspect of the nucleus.
Origin of the Theory of the Nucleus.—Rutherford was led to enunciate the nuclear theory of the atom as a result of a study of the scattering of a-particles by matter. The a-particles are atoms of helium, carrying a positive charge 2e (where e is the magnitude of the electronic charge), which are shot off spon taneously by certain radioactive bodies. (See RADIOACTIVITY.) The a-rays from a given radioactive element are homogeneous in velocity (except for a few longer range particles present in certain cases), and thus provide a very valuable instrument for investi gation. Those from Radium C, for instance, which have been chiefly, though not exclusively, used for the scattering experi ments, are discharged with a velocity of about 1.922 X cm./sec., and an energy of 1•224 x i ergs. Swift a-particles can be detected by their action on a phosphorescent screen. If a surface be covered with phosphorescent zinc sulphide, which is the substance usually employed, and the soft glow produced in the sulphide by the near presence of a radioactive substance be examined with a low power microscope, it is found that the light is not uniform, but made up of evanescent specks of light, which are referred to as scintillations. These scintillations are due to the impact of a-rays on the screen. (See RADIOACTIVITY.) With suit able precautions every a-particle can be made to produce a scintil lation so long as its energy has not been too much diminished by passage through matter before the impact. A phosphorescent screen placed at right-angles to the path of a pencil of a-rays gives us, then, a method of ascertaining how many a-particles fall in a given time within a given small area, that is, proceed in a given direction, and it is by the scintillation method that the scattering of a-particles has been investigated. A narrow beam of a-rays of homogeneous velocity, say from a source of Radium C, produces a sharp spot on the screen. If a very thin metal foil be introduced in the path of the beam the spot loses its sharp outline and becomes diffuse, and a study of the scintillations makes it possible to say what fraction of the particles has been deflected through a given angle by the passage through the foil.