from the Physico- Chemical Standpoint. 327 



(3) That the "valency" electrons are easily removable in 

 chemical reactions ; whilst the /3-ray electrons are 

 ejected spontaneously and cannot be withdrawn from 

 the atom by any process under our control. 



{4) That the atomic number of an atom can be altered by 

 either an a- or a /3-ray change. 



(5) That in ana-ray change the ejected material is always* 



a helium atom carrying two positive charges. 



(6) That a change in the valency of an element produced 



by chemical means alters the chemical properties of 

 that element in a manner similar to that which is 

 observed when a /3-ray is ejected ; but that there is a 

 difference in degree between the effects produced in 

 the two cases. 

 {7) That certain atoms possessing different atomic weights 

 show the same chemical properties, whilst other atoms 

 having atomic weights identical with one another 

 exhibit totally different chemical characteristics. 



The model atom which will now be described covers 

 these points; and it appears to possess certain features of 

 novelty. 



At the centre of the structure is a group cf negative 

 electrons travelling in closed orbits which, for the sake of 

 clearness, may be assumed to be circular. Closely sur- 

 rounding this negative group lies another series of orbits 

 occupied by positive electrons f which, in some cases, are 

 associated with negative electrons in a manner to be dealt 

 with later. These orbits are assumed to be circular also ; their 

 extreme diameter may be taken, according to Rutherford's 

 view}, as not being greater than 10~ 12 cm.; and, as in the 

 Rutherford atom, the mass of the system is assumed to be 

 concentrated in this portion. Further still from the centre, 

 other electrons move in orbits of an elliptical character, the 

 ellipses being much elongated, so that the electrons travel 

 in paths like those of comets in the solar system. The 



* The neon discovered in certain mineral springs is not yet proved 

 to be of radioactive origin ; and it is therefore left out of account 

 here. 



t This assumption as to the relative positions of the positive and 

 negative zones is made purely for convenience. The general argument 

 is not affected by an inversion of their positions, or even by assuming 

 that they form a kind of double-star system. 



% Rutherford, ' Radioactive Substances and their Radiations/ p. 621 

 (1913). 



