4i6 



Supplement to ''Nature" September 15, 1923 



I 



and this has been confirmed by direct experiment. 

 Dr. Chadwick and I have observed that swift hydrogen 

 niitlci arc released from the elements l>oron, nitrogen, 

 fluorine, sodium, aluminium, and phosphorus when 

 they are lM)mlmrded by swift tt-{)artic:lcs, and there is 

 little room for doubt that these hydrogen nuclei form 

 an essential part of the nuclear structure. The speed 

 of ejection of these nuclei depends on the velocity 

 of the a-particle and on the element bombarded. It 

 is of interest to note that the hydrogen nuclei are 

 liberated in all directions, but the speed in the back- 

 ward direction is always somewhat less than in the 

 direction of the o-particle. Such a result receives a 

 simple explanation if we suppose that the hydrogen 

 nuclei are not built into the main nucleus but exist 

 as satellites probably in motion round a central core. 

 There can be no doubt that bombardment by 

 a-particles has effected a veritable disintegration of 

 the nuclei of this group of elements. It is significant 

 that the liberation of hydrogen nuclei only occurs in 

 elements of odd atomic number, namely, 5, 7, 9, 11, 13, 

 15, the elements of even number appearing quite un- 

 affected. P'or a collision of an a-particle to be effective, 

 it must either pass close to the nucleuS or actually 

 penetrate its structure. The chance of this is ex- 

 cessively small on account of the minute size of the 

 nucleus. For example, although each individual 

 a-particle will pass through the outer structure of more 

 than 100,000 atoms of aluminium in its path, it is 

 only about one a-particle in a million that gets close 

 enough to the nucleus to effect the liberation of its 

 hydrogen satellite. 



This artificial disintegration of elements by a-particles 

 takes place only on a minute scale, and its observation 

 has only been possible by the counting of individual 

 swift hydrogen nuclei by the scintillations they produce 

 in zinc sulphide. 



These experiments suggest that the hydrogen nucleus 

 or proton must be one of the fundamental units which 

 build up a nucleus, and it seems highly probable that 

 the helium nucleus is a secondary building unit com- 

 posed of the very close union of four protons and two 

 electrons. The view that the nuclei of all atoms are 

 ultimately built up of protons of mass nearly one and 

 of electrons has been strongly supported and extended 

 by the study of isotopes. It was early observed that 

 some of the radioactive elements which showed distinct 

 radioactive properties were chemically so alike that 

 it was impossible to effect their separation when mixed 

 together. Similar elements of this kind were called 

 " isotopes " by Soddy, since they appeared to occupy 

 the same place in the periodic table. For example, a 

 number of radioactive elements in the uranium and 

 thorium series have been found to have physical and 

 chemical properties identical with those of ordinary 

 lead, but yet to have atomic weights differing from 

 ordinary lead, and also distinctive radioactive pro- 

 perties. The nuclear theory of the atom offers at 

 once a simple interpretation of the relation between 

 isotopic elements. Since the chemical properties of 

 an element are controlled by its nuclear charge and 

 little influenced by its mass, isotopes must correspond 

 to atoms with the same nuclear charge but of different 

 nuclear mass. Such a view also offers a simple 

 explanation why the radioactive isotopes show different 



radioactive properties, for it is to be anticipated that 

 the stability of a nucleus will be much influenced by 

 its mass and arrangement. 



■ Our knowledge of isotopes has been widely extended 

 in the last few years by Aston, who has devised an 

 accurate direct method for showing the presence of 

 i.sotopes in the ordinary' elements. He has found that 

 some of the elements are " pure " — i.t. consist of at'?!- 

 of identical mass — while others contain a mr 

 two or more isotopes. In the case of the 

 elements, the atomic mass, as ordinarily measured by 

 the chemist, is a mean value depending on the atomic 

 masses of the individual isotoyjes and their relative 

 abundance. These investigations have not only shown 

 clearly that the number of distinct species of atoms b 

 much greater than was supposed, but have also brought 

 out a relation Ijetween the elements of great interest 

 and importance. The atomic masses of the isotopes 

 of most of the elements examined have been found, 

 to an accuracy of about one in a thousand, to be 

 whole numbers in terms of oxygen, 16. This indicates 

 that the nuclei are ultimately built up of protons of 

 mass very nearly i and of electrons. It is natural 

 to suppose that this building unit is the hydrogen 

 nucleus, but that its average mass in the complex 

 nucleus is somewhat less than its mass in the free state 

 owing to the close packing of the charged units in the 

 nuclear structure. We have already seen that the 

 helium nucleus of mass 4 is probably a secondary unit 

 of great importance in the building up of many atoms, 

 and it may be that other simple combinations of 

 protons and electrons of mass 2 and 3 occur in the 

 nucleus, but these have not been observed in the free 

 state. 



While the mass of the majority of the isotopes are 

 nearly whole numbers, certain cases have been observed 

 by Aston where this rule is slightly departed from. 

 Such variations in mass may ultimately prove of great 

 importance in throwing light on the arrangement and 

 closeness of packing of the protons and electrons, and 

 for this reason it is to be hoped that it may soon prove 

 possible to compare atomic masses of the elements 

 with much greater precision even than at present. 



While we may be confident that the proton and the 

 electron are the ultimate units which take part in the 

 building up of all nuclei, and can deduce with some 

 certainty the number of protons and electrons in the 

 nuclei of all atoms, we have little, if any, information 

 on the distribution of these units in the atom or on 

 the nature of the forces that hold them in equilibrium. 

 While it is known that the law of the inverse square 

 holds for the electrical forces some distance from the 

 nucleus, it seems certain that this law breaks down 

 inside the nucleus. A detailed study of the collisions 

 between o-particles and hydrogen atoms, where the 

 nuclei approach ver)- close to each other, shows that 

 the forces between nuclei increase ultimately much 

 more rapidly than is to be expected from the law of the 

 inverse square, and it may be that new and unexpected 

 forces may come into importance at the ver\- small 

 distances separating the protons and electrons in the 

 nucleus. Until we gain more information on the 

 nature and law of variation of the forces inside the 

 nucleus, further progress on the detailed structure of 

 the nucleus may be difficult. At the same time, there 



