212 



NA TURE 



[August i i, 1923 



passage through atoms, by the existence of a very 

 minute highly charged nucleus at the centre of the 

 atom, the rest of the atom being occupied by separate 

 charges of opposite sign equal in number to the nuclear 

 charge. For such an atom scattering should be 



iNTH£ PERIOD OF 

 AVERAGE LIFE 



I 

 I 



CHANCE 



/fAY CHANCl 



UNITS OF + NUCLEAR CHARGE-*- 



■ ' ' ■ 



Fig. I. — Radio-elements and the periodic law. All elements in the same vertical column are isotopes. 



proportional to the square of the nuclear charge. 

 Experiment showed that scattering was approximately 

 proportional to the square of the atomic weight. 

 So that it looked as if, as in the a-particle itself, there 

 existed one unit of nuclear charge to each two units 

 of atomic weight. This would make the nuclear 

 charge of uranium of atomic weight about 240, 120 + . 



NO. 2806, VOL. 112] 



Since the a-particle carries two positive charges 

 and the /i-particle one negative, the obvious inference 

 from the figure is that the successive places in the 

 Periodic Table correspond with unit difference in the 

 intra-atomic charge. This view, and also that each 

 unit of charge corresponded to two units of mass, 

 had been suggested independently by van der Broek 

 in 191 1. At first he tried to stretch the Periodic Table 

 to make it accommodate 120 places. But in 1913 he 

 pointed out that the experimental results for scattering 

 were completely in accord with his own view (that 

 the number of the place or atomic number is the same 

 as the intra-atomic charge) on the existing Periodic 

 Table, which accommodates some 90 elements. It 

 would not be inconsistent with his other view (that 

 the nuclei of the heavy elements are made up of helium 

 nuclei) if there were electrons in the nucleus as well 

 as in the outside shell. Thus uranium in the 90th 

 place would have to have, in addition to the 60 helium 

 nuclei in its nucleus, to account for its weight, 30 

 electrons, to account for its charge of 90 -f- . 



The existence of electrons as well as positive charges 

 in the atomic nucleus was also postulated by Bohr 

 to explain the emission of /i-rays, for on his theory' 

 the electrons in the external shell form a 

 stable configuration and could only be dis- 

 lodged by the expenditure of work. 



The Periodic Law generalisation practi- 

 cally settled this question, /i^-ray changes 

 are no less transmutational than a-ray 

 changes, and are sharply to be distinguished 

 from the numerous processes, such as 

 friction, chemical change, 

 action of ultra-violet 

 light, and incandescence, 

 during which electrons 

 are detached from atoms. 

 The effect on the chem- 

 ical character produced 

 by the expulsion of one 

 a-particle is exactly un- 

 done by the expulsion 

 of two /i-particles, and 

 the product becomes 

 isotopic with the original 

 parent. This means that 

 both a- and /i-particles 

 must be expelled from 

 the nucleus and that iso- 

 topes are elements the 

 atoms of which have the 

 same nett nuclear charge ; 

 i.e. the same excess num- 

 ber of positive over nega- 

 tive charges in the 

 nucleus, but different 

 numbers of positives and 

 negatives reckoned separ- 

 ately. For such systems 

 the electronic shell would be identical, and so the iden- 

 tity of the chemical and spectroscopic character is ex 

 plained. Also the atomic volume is the same; that 

 is, the density must be proportional to the atomic 

 weight. 



We were able to get an interesting confirmation of 

 this view. In the change of uranium X^ to uranium II 



239 



