ELECTRICAL STRUCTURE OF MATTER RUTHERFORD 171 



the atomic number of an element was a measure of its nuclear 

 charge, and the correctness of this deduction has been recently veri- 

 fied by Chadwick by direct experiments on the scattering of a 

 particles. Moseley's discovery is of fundamental importance, for it 

 not only fixes the number of electrons in all the atoms, but shows 

 conclusively that the properties 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 excep- 

 tions all atomic numbers between hydrogen 1 and uranium 92 would 

 correspond to Imown elements. The great power of Moseley's law 

 in fixing the atomic number of an element is well illustrated by the 

 recent discovery by Coster and Hevesy in Copenhagen of the miss- 

 ing 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 struc- 

 ture of the atom falls naturally into two great divisions: One, the 

 arrangement 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 de- 

 pends. On the nuclear theory the hydrogen atom is of extreme sim- 

 plicity, consisting of a singly-charged positive nucleus with only 

 one attendant electron. The position and motions of the single elec- 

 tron must account for the complicated optical spectrum, and Avhat- 

 ever 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 neces- 

 sary. 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 radiation is emitted in definite units or quanta, in which the 

 energy E of a radiation is equal to Av where v is the frequency of 

 the radiation measured by the ordinary methods and A 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 cor- 

 responding to the X rays or the y rays from radium. 



Time does not allow me to discuss the underlying meaning of the 

 quantum theorj^ or the difficulties connected with it. Certain aspects 



