PHYSICS 



elements. A very clear insight into the nature 

 of the products of these transformations is obtained 

 when they are considered from the point of view 

 of the charge of the nucleus of the atom. An 

 a ray consists of a helium nucleus and has a 

 positive charge equal to the charge of two negative 

 electrons. For the sake of brevity let us refer to 

 the electronic charge as the unit charge ; it is, 

 in fact, the natural unit of electric charge. Then 

 the expulsion of an a particle will reduce the 

 nuclear charge by two units. On the other hand 

 the expulsion of an electron, which is negatively 

 charged, will increase the nuclear charge by a 

 single unit. Inasmuch as we have seen that the 

 chemical properties and the chief physical pro- 

 perties of the elements are determined by the 

 magnitude of the nuclear charge, each such ex- 

 pulsion of an a or (3 particle will in general result 

 in the generation of an atom of a new element. 

 Furthermore, the ejection of an a particle will 

 diminish the atomic weight of an atom by 4 units, 

 the atomic weight of helium being very close to 4. 

 The expulsion of a (3 particle, on the other hand, 

 will leave the atomic weight of the atom unaffected. 

 One result of this is that the expulsion of a fi par- 

 ticle will generate an atom of an element whose 

 chemical properties are different from those of the 

 parent atom, but whose atomic weight is the same 

 as that of the parent atom. 



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