Supplement to ''Nature^' September 15, 1923 



41 



fixes the number of the outer planetary electrons in 

 the neutral atom. In addition, since these outer 

 electrons are in some way held in equilibrium by the 

 attractive forces from the nucleus, and, since we are 

 confident from general physical and chemical evidence 

 at all atoms of any one element are identical in their 

 temal structure, it is clear that their arrangement 

 d motion must be governed entirely by the magnitude 

 the nuclear charge. Since the ordinary chemical 

 d physical properties are to be ascribed mainly to 

 e configuration and motion of the outer electrons, it 

 'ollows that the properties of an atom are defined by 

 a whole number representing its nuclear charge. It 

 thus becomes of great importance to determine the 

 value of this nuclear charge for the atoms of all the 

 elements. 



Data obtained from the scattering of a-particles, 

 and also from the scattering of X-rays by light elements, 

 indicated that the nuclear charge of an element was 

 numerically equal to about half the atomic weight in 

 terms of hydrogen. It was fairly clear from general 

 evidence that the hydrogen nucleus had a charge one, 

 and the helium nucleus (the a-particle) a charge two. 

 t this stage another discovery of great importance 

 Tovided a powerful method of attack on this problem, 

 he investigation by Laue on the diffraction of 

 rays by crystals had shown definitely that X-rays 

 were electromagnetic waves of much shorter wave- 

 length than light, and the experiments of Sir William 

 Bragg and W. L. Bragg had provided simple methods 

 for studying the spectra of a beam of X-rays. It was 

 found that the spectrum in general shows a continuous 

 background on which is superimposed a spectrum of 

 l)right lines. At this stage H. G. J. Moseley began a 

 research with the intention of deciding whether the 

 properties of an element depended on its nuclear charge 

 rather than on its atomic weight as ordinarily supposed. 

 For this purpose the X-ray spectra emitted by a number 

 of elements were examined and found to be all similar 

 in type. The frequency of a given line was found to 

 \ary very nearly as the square of a whole number which 

 varied by unity in passing from one element to the 

 next. Moseley identified this whole number with the 

 atomic or ordinal number of the elements when arranged 

 in increasing order of atomic weight, allowance being 

 made for the known anomalies in the periodic table and 

 for certain gaps corresponding to possible but missing 

 elements. He concluded that the atomic number of 

 an element was a measure of its nuclear charge, and the 

 correctness of this deduction has been recently verified 

 by Chadwick by direct experiments on the scattering of 

 a-particles. Moseley 's discovery is of fundamental im- 

 portance, for it not only fixes the numl)er of electrons in 

 all the atoms, but also shows conclusively that the pro- 

 perties of an atom, as had been surmised, are determined 

 not by its atomic weight but by its nuclear charge. A 

 relation of unexpected simplicity is thus found to hold 

 between the elements. . No one could have anticipated 

 that with few exceptions all atomic numbers between 

 hydrogen i, and uranium 92, would correspond to 

 known elements. The great power of Moseley's law in 

 fixing the atomic number of an element is well illus- 

 trated by the recent discovery by Coster and Ilevesy in 

 Copenhagen of the missing element of atomic number 

 72, which they have named " hafnium." 



Once the salient features of the structure of atoms 

 have been fixed and the number of electrons known, 

 the further study of the structure of the atom falls 

 naturally into two great divisions : one, the arrange- 

 ment of the outer electrons which controls the main 

 physical and chemical properties of an element, and the 

 other, the structure of the nucleus on which the mass 

 and radioactivity of the atom depend. On the nuclear 

 theory the hydrogen atom is of extreme simplicity, con- 

 sisting of a singly-charged positive nucleus with only 

 one attendant electron. The position and motions of 

 the single electron must account for the complicated 

 optical spectrum, and whatever physical and chemical 

 properties are to be attributed to the hydrogen atom. 

 The first definite attack on the problem of the electronic 

 structure of the atom was made by Niels Bohr. He 

 saw clearly that, if this simple constitution was assumed, 

 it is impossible to account for the spectrum of hydrogen 

 on the classical electrical theories, but that a radical 

 departure from existing views was necessary. For this 

 purpose he applied to the atom the essential ideas of 

 the quantum theory which had been developed by 

 Planck for other purposes, and had been found of great 

 service in explaining many fundamental difficulties in 

 other branches of science. On Planck's theory, radia- 

 tion is emitted in definite units or quanta, in which 

 the energy E of a radiation is equal to }iv where v is the 

 frequency of the radiation measured by the ordinary 

 methods and h a universal constant. This quantum of 

 radiation is not a definite fixed unit like the atom of 

 electricity, for its magnitude depends on the frequency 

 of the radiation. For example, the energy of a 

 quantum is small for visible light, but becomes large 

 for radiation of high frequency corresponding to the 

 X-rays or the y-rays from radium. 



Time does not allow me to discuss the underlying 

 meaning of the quantum theory or the difficulties con- 

 nected with it. Certain aspects of the difficulties were 

 discussed in the presidential address before this Associa- 

 tion by Sir Oliver Lodge at Birmingham in 1913. It 

 suffices to say that this theory has proved of great value 

 in several branches of science, and is supported by a 

 large mass of direct experimental evidence. 



In applying the quantum theory to the structure of 

 the hydrogen atom Bohr supposed that the single 

 electron could move in a number of stable orbits, con- 

 trolled by the attractive force of the nucleus, without 

 losing energy by radiation. The position and character 

 of these orbits were defined by certain quantum rela- 

 tions depending on one or more whole numbers. It 

 was assumed that radiation was only emitted when the 

 electron for some reason was transferred from one stable 

 orbit to another of lower energy. In such a case it was 

 supposed that a homogeneous radiation was emitted of 

 frequency v determined by the quantum relation Y. — hv 

 where E was the difference of the energy of the electron 

 in the two orbits. Some of these possible orbits are 

 circular, others elliptical, with the nucleus as a focus, 

 while if the change of mass of the electron with velocity 

 is taken into account the orbits, as Sommerfeld showed, 

 depend on two quantum numbers, and are not closed, 

 but consist of a nearly elliptical orbit slowly rotating 

 round the nucleus. In this way it is possible not only 

 to account lor the series relations between the bright 

 lines of the hydrogen spectrum, but also to explain the 



