Nucleus

A striking feature of these photographs is that, in the disrup tion cases, there is no track corresponding to the a-particle after collision. This indicates that the a-particle enters into the nucleus and remains there, forming a nucleus of mass 14-1+4=17, and of nuclear charge 7-1+2 = 8. This would be an isotope of oxy gen, whose existence has not been detected by the ray spectro graph. (See ISOTOPES.) However, ray-track photographs of Har kins and Ryan, and of Akiyama, taken in air, show, in the case of disintegration, a track of the a-particle after the disrupting col lision, indicating that the a-particle does not always stick in the nucleus. The reason for this difference of behaviour is not yet explained.

Stability of Nucleus. Atomic Energy.

The evidence which has been considered clearly shows that the nuclei of all atoms are ultimately built up of electrons and protons in intimate combination. It is clear that combinations containing more than a certain number of these component parts are not stable, for the very heavy nuclei pertain to radioactive atoms, and break up spontaneously. However, of the radioactive nuclei the heaviest are not the most unstable, for Uranium I. and Uranium II., both of atomic number 92, and of atomic weight 238.2 and 234.2 respectively, have very large half-value periods (see RADIO ACTIVITY), that is, break down very infrequently, while Actinium C' and Radium C', both of atomic number 84, and of atomic weight 210 and 214 respectively, are extraordinarily unstable. The explanation of the relative instability of the radioactive nuclei in terms of their structure or composition is still to seek. Fajans, however, has enunciated a rule that all nuclei of atomic mass 4n, where n is a whole number, and of odd nuclear charge, are very unstable. The only radioactive nuclei which fall in this class are those of Thorium C", Thorium C, and Mesothorium II., which are all a-radiators of comparatively short life. Nuclei of the general composition specified are not found among the other elements, so that the rule, which is purely empirical, does seem to have a general application.

Evidence of relative stability among the nuclei of non-radio active elements has been sought by Harkins in the comparative abundance in which different kinds of atoms occur in nature, the assumption being that elements of rare occurrence are com paratively unstable. The commonest elements, both in meteorites. which may be regarded as samples from extra-terrestrial sources, and in the earth, as far as we can judge from geological evidence, are oxygen, silicon, magnesium, and iron, all comparatively light elements of even atomic number. Taking the figures as a whole, elements of even atomic number are much commoner than ele ments of odd atomic number. Further consideration of the conjunction of atomic weights and atomic numbers in individual elements leads to the conclusions that the number of nuclear electrons is generally even, or nuclear electrons tend to occur in pairs. There are a fair number of empirical rules of this nature,

which enunciate generalities, but no laws of universal application. This aspect of the subject still awaits a co-ordinating theory.

The a-particle itself is a particularly stable entity, consisting of four protons and two electrons: the two electrons which it picks up when it becomes a neutral helium atom do not, of course, form any part of the nucleus, but are, relatively speaking, very distant and very loosely held. Evidence of the stability is ob tained by consideration of the peculiar fact that, while the mass of the two electrons is negligible, the mass of the helium nucleus is not exactly equal to that of four protons, for the atomic weight of helium in terms of oxygen as 16, is 4.00, while the atomic weight of hydrogen is 1.0077. We might expect the atomic weight of helium to be 4-0308: the difference between this number and the actual number 4.00 is called the mass defect.

The explanation of the mass defect is to be sought in the electromagnetic nature of mass. If an electric charge be concen trated in a very small space it requires a force to produce an acceleration of its movement, or, in other words, it possesses inertia. The smaller the volume into which it is crowded, the greater the inertia. Making, for the sake of precision, the very simple, and somewhat improbable, assumption that elementary charges behave as small charged spheres, the electron must have a diameter of 3.8X cm., the proton a diameter of 2X cm. to give the required masses. Even without this assumption we may say, quite generally, that the electromagnetic mass depends not only on the charge but on the capacity of the system. If we bring two small charged spheres of opposite sign close together the capacity of the system is not the sum of that of its parts, considered separately, so that the electromagnetic mass of the two together is less than the sum of the two separately. In general, whenever we pack protons close together with electrons we might anticipate a diminution of mass due to this close packing. The fact that the mass of a nucleus is always less than the mass to be anticipated, by simple addition, from the number of protons it contains is said to be due to the "packing effect." (See ISO TOPES, Packing Fraction.) Quite apart from any mechanism, such as that just considered, we can get an estimate of the stability of the helium nucleus from the mass defect, by taking into account Einstein's relation between mass and energy. (See RELATIVITY.) This relation is where m is the mass, E the energy, c the velocity of light, which is 3 X Io'' cm./sec. If a system loses mass it loses energy to an amount represented by this formula. For the helium nucleus the energy per gram molecule, i.e., per 4 grams of the gas, is E= .o3o8 X 9 X 10" = .28 X 10" ergs = 7 X 1o" gram calories.