184 



NA JURE 



[August, 5, 1922 



it was deduced that the actual magnitude of the 

 nuclear charge of an atom in fundamental units is 

 equal to the atomic or ordinal number when the 

 elements were arranged in order of increasing atomic 

 weight. On this view, the nuclear charge of hydrogen 

 is 1. of helium 2, lithium 3, and so on up to the heaviest 

 element uranium, of charge 92. It has been found that 

 between these limits, with few exceptions, all nuclear 

 charges are represented by known elements. 



Tins relation, found by Moseley, between the atoms 

 of the elements is of unexpected simplicity and of 

 extraordinary interest. The properties of an atom 

 are defined by a whole number which varies by unity 

 in passing from one atom to the next. This number 

 not only represents the ordinal number of the elements, 

 but also the magnitude of the charge of the nucleus 

 and the number of outer electrons. It could scarcely 

 have been anticipated that, possibly with few excep- 

 tions, all nuclear charges between 1 and 92 would 

 represent elements found on the earth. With the 

 exception of the radio-active elements, the atoms are 

 all stable for intervals represented by millions of years. 

 The atomic weight of an element is not nearly so funda- 

 mental a property of the atom as its nuclear charge, 

 for its weight depends upon the inner structure of 

 the nucleus, which may be different for atoms of the 

 same nuclear charge. 



The most definite information we have of the struc- 

 ture of the nucleus has been obtained from a study 

 of the modes of disintegration of the radio-active 

 atoms. In the great majority of cases the atoms 

 break up with the expulsion of a single u-particle 

 which represents the doubly charged nucleus of the 

 helium atom ; in other cases a swift /i-ray or electron 

 is liberated. There can be no doubt that these particles 

 are liberated from the nuclei of the radio-active atoms. 

 This is clearly shown by the variation of the atomic 

 numbers of the successive elements in the long series 

 of transformations of uranium and thorium (see Fig. 1). 

 The expulsion of an a-particle lowers the nuclear 

 charge of the atom by two units and its mass by four, 

 while the expulsion of an electron raises it by one. 

 On this simple basis we can at once deduce the atomic 

 number and, consequently, the general chemical prop- 

 erties of the long series of radio-active elements. In 

 this way we can understand at once the appearance 

 in the radio-active series of isotopes, i.e. elements of 

 the same nuclear charge but different atomic masses. 



The existence of isotopic elements was first brought 

 to light from a study of the radio-active elements. 

 For example, radium-B, radium-Z) and the end product, 

 uranium-lead, are isotopes of lead of nuclear charge 

 82, but of masses 214, 210, and 206 respectively. As 

 regards ordinary chemical and physical properties, 

 thev are indistinguishable from one another, differing 

 only 111 properties that depend on the nucleus, namely, 

 atomic mass and radio-activity. For example, radium- 

 B and radium-D both emit /^-rays, but with different 

 velocities, while their average life is widely different. 

 Uranium-lead, on the other hand, is non-radioactive. 

 .Munv similar examples can be taken from the thorium 

 and actinium series of elements. These illustrations 

 show clearly that elements may have almost identical 



NO. 2753, VOL. I IO] 



physical and chemical properties and yet differ markedly 

 in the mass and structure of their nuclei. 



From the radio-active evidence it seems clear that 

 the nuclear structure contains both helium nuclei and 

 electrons. In the uranium-radium series of transforma- 

 tions, eight helium nuclei are emitted and six electrons, 

 and it is natural to suppose that the helium nuclei and 

 electrons that are ejected act as units of the nuclear 

 structure. It is clear from these results that the nuclear 

 charge of an element is the excess of the positive 

 charges in the nucleus over the negative. It is a 

 striking fact that no protons (H nuclei) appear to be 

 emitted in any of the radio-active transformations, but 

 only helium nuclei and electrons. 



Some very definite and important information on 

 the structure of nuclei has been obtained by Aston in 

 his experiments to show the existence of isotopes in 



.. . Ur.I VrX l UlX 2 Ur.n Ionium Radium Eman. 



Atm.Wt.Z38 234 234 234 230 226 222 



c 



@K (§>— %-* @r-» ©-* ®-~ @ 



Rad.A Rad.B Rad.C Rad.D Rad.E Rad.F lead 



218 214 214 210 210 210 206 



Urajiium-radium series 



the ordinary stable elements by the well-known 

 positive-ray method. He found that a number of 

 the elements were simple and contained no isotopes. 

 Examples of such " pure " elements are carbon, 

 nitrogen, oxygen, and fluorine. It is significant that 

 the atomic weights of these elements are nearlv whole 

 numbers in terms of 0=i6; on the other hand, 

 elements such as neon, chlorine, krypton, and many 

 others consisted of mixtures of two or more isotopes 

 of different atomic masses. Aston found that within 

 the limit of error — about i in iooo — the atomic weights 

 of these isotopes were whole numbers on the oxygen 

 scale. This is a very important result, and suggests 

 that the nuclei of elements are built up by the addition 

 of protons, of mass nearly one, in the nuclear com- 

 bination. 



[Experimental evidence was here given of the libera- 

 tion of protons from the elements boron, nitrogen, 

 fluorine, sodium, aluminium, and phosphorus. See 

 Nature of May 6, p. 584.] 



From the radio-active evidence, we know that the 

 nuclei of heavy atoms are built up, in part at least, 

 of helium nuclei and electrons, while it also seems 

 clear that the proton can be released from the nuclei 

 of certain light atoms. It is, however, very natural 

 to suppose that the helium nucleus which carries two 

 positive charges is a secondary building unit, composed 

 of a close combination of protons and electrons, namely, 

 4 protons and 2 electrons. 



From the point of view of simplicity, such a con- 

 ception has much in its favour, although it should be 

 mentioned that it seems at the moment impossible 

 to prove its correctness. If, however, we take this 

 structure of the helium nucleus as a working hypothesis] 

 certain very important consequences follow. On the 



