Atomic Nucleus and the Law of Force. 735 



bv a distribution of electrons extending over a distance 

 comparable with the diameter of the atom, as usually 

 understood. 



The physical and chemical properties of an element are 

 determined by the charge on the nucleus, for this fixes the 

 number and arrangement of the external electrons, on which 

 these properties mainly depend. The mass of the nucleus 

 influences the arrangement of the electrons only to a very 

 small degree. The nuclear charge is thus the fundamental 

 constant of the atom and the question of its actual magni- 

 tude of great importance for the development of atomic 

 theory. 



In the paper referred to above, Sir Ernest Rutherford 

 showed — assuming that the electric forces between the 

 nucleus and an « particle passing close to it varied ac- 

 cording to the inverse square law — that the a particle 

 would describe a hyperbolic path, and obtained the rela- 

 tions connecting the fraction of a particles scattered 

 through any angle with the charge on the nucleus and 

 the velocity of the a particle. From some observations 

 of Greiger and Marsden* on the reflexion of « particles he 

 deduced that the charge on the nucleus of an atom is 

 roughly |A . e, where A is the atomic weight and e the 

 electronic charge. 



In their experiments on the scattering of a. particles, 

 which proved conclusively the truth of the nuclear theory, 

 (xeiger and Marsden showed that the charge on the nucleus 

 was equal to lA.e to within about 20 per cent. The large 

 error is due to the fact that the scattered particles formed 

 such a small fraction of the original beam that different 

 methods of measurement had to lie employed in the two 

 cases. 



Later, van den Broek f suggested that the nuclear charge 

 might be equal to the atomic number of the element, 

 i. e. the number of the element when all the elements are 

 arranged in order of increasing atomic weight. This proved 

 to be in good agreement with the experiments on scattering, 

 but the importance of the atomic number was first shown 

 by Moseley's work on the X-ray spectra of the elements. 

 Moseley i found that the X-ray spectra of the elements 

 depended on the square of a number which increased by 

 unity in passing from one element to the next of higher 

 atomic weight. This number was not exactly equal to 



* Geiger and Marsden, Phil. Mag-, xxv. p. 004 (1913). 



t Van den Broek, Phys. Zeit. xiv. p. 32 (1913). 



% Moseley, Phil. Mag. xxvi. p. 1024 (1913) ; xxvii. p. 70:5 (1914). 



