Nucleus

These experiments confirm and extend earlier results found with aluminium and magnesium, which indicated that the inverse-square law held to within one per cent or so for distances greater than 3 X cm. Speaking generally, then, we may say that the radius of nuclei, apparently not very different for different ele ments, is of the order 2X cm., in the sense that at this dis tance there is a rapid change in the law of force, while it is possible to force a proton to within a distance of 4X Is:Yu cm. from the centre of an a-particle.

Very interesting results were obtained by Rutherford and Chadwick in their experiments of 1927 on the scattering of a-particles by helium. The difficulty presented by the fact that a gas like helium cannot be obtained in a thin plate was sur mounted by the use of diaphragms, which isolated an incident and a scattered pencil of rays in such a way that all particles reaching the phosphorescent screen must have come from a small annulus of the gas. The experiments were complicated by the fact that both scattered particles and struck nuclei are of the same nature, but skilful interpretation of the results allowed definite conclusions to be drawn. It was deduced that, for central collision, departure from the inverse-square law occurs at distances of about 3.5X 10" cm. between the centres, while for glancing collisions (small angles of scattering) the departure occurs at about 14 cm. This indicates a plate-like form for the a-particles, confirming the conclusions reached by Chadwick and Bieler from an exhaustive study of the oollision of the a-particles with hydrogen nuclei.

It does not appear that these results are to be attributed to the departure from the classical laws of mechanics embodied in the new wave-mechanics (see QUANTUM THEORY), but rather that they are best explained on ordinary mechanical principles applied to a field of force departing from the inverse-square. Rutherford has suggested tentatively that the departures have their physical origin in a definite magnetic moment associated with the helium nucleus, which produces a magnetic force becoming prominent at distances of the order of 4 X cm. The turning couple due to two such magnetic moments would explain the orientation of the particles during collision. Of course a magnetic moment must be assumed for the proton itself, the moment of the helium nucleus being a resultant of the moments of its components. The introduc tion of a consideration of magnetic forces, so long neglected in the internal physics of the atom, seems likely to prove fruitful in the future.

We are faced with the paradox that, while the nucleus must, as we shall see, contain electrons (in the case of a heavy element like gold a great many electrons) yet the size of the nucleus is not much larger than that of the electron, for which the diameter is generally given as 4 X cm. The size of the electron, however, is far

less directly determined than that of the nucleus. The estimate just quoted is based upon the assumption that the mass is entirely electromagnetic, and is that which is produced by a distribution of the electronic charge throughout the volume of a small sphere. The electromagnetic mass of such a sphere, moving at slow speeds, is where e is the charge in electromagnetic units, and a is the radius. If instead we assume that the charge is spread over the surface of the sphere we have Where so little is known it does not matter which formula we adopt. The assumption of such a formula is our only way of obtaining an estimate of the size of the electron, for there is no way at present known of investigating the field of force round an electron, which is the only really significant thing. The paradox of the approximate equality of electronic and nuclear size is therefore really explained by the fact that little significance can be attached to the value given for the radius of the electron, and we may further say that, whatever the radius of a free electron, the radius of an electron in close combination with other electrons and protons may be quite different.

The Structure of the Nucleus.

The constitution of the nucleus has been the subject of much speculation. The positive charge is ultimately due to the presence in the nucleus of protons, whose mass is that of the hydrogen atom and whose positive charge is r in terms of the electronic charge. If these were the only constituents the atomic number of any nucleus should be equal to its mass in terms of hydrogen as 1, but it is well-known that the atomic number—the net positive charge—is always markedly less than this, being usually less than half of the atomic weight. This indicates that the nucleus itself must contain a number of electrons, whose presence diminishes the positive charge, but leaves the mass practically unaltered. The expulsion of 13-particles of very high energy might be taken to indicate the presence of such electrons in the nuclei of radioactive elements, but the occurrence of the by itself would not be con vincing, as they might, without further experiments, be supposed to come from the outer parts of the atom, being ejected by a sec ondary action of nuclear y-rays. However, it is found that strong j3-radiators are transformed to elements whose chemical behaviour indicates a place in the periodic table corresponding to an increase of atomic number by unity, that is, to a gain of one unit of positive charge by the nucleus. This is equivalent to the loss of one electron by the nucleus, so the behaviour of radioactive ele ments affords strong evidence for the presence of electrons in the nucleus.