628 SCIENCE PROGRESS 



more and more complex atomic models. It will be evident 

 at once that, as this process goes on, we have periodic changes 

 in the type of distribution involved. Thus, the atom con- 

 taining one electron has this electron at the centre. The atom 

 containing six electrons has again one at the centre, the re- 

 mainder forming the outer ring of the two-ring system. The 

 atom with seventeen electrons is three-ring, one electron being 

 in the centre, then a ring of five, and lastly a ring of eleven. 

 The atom of thirty-two electrons is four-ring, one electron in 

 the centre, then a ring of five, then a ring of eleven, and lastly 

 a ring of fifteen. This recurrence of the same type — one 

 central electron — at various intervals as the atomic weight 

 increases, leads us to expect a recurrence of similar properties. 

 This is the analogue of the Periodic Law. 



Although this demonstration of periodicity in properties 

 was a very great step, the question of the distribution of the 

 positive charge was still left quite unsettled. More recently, 

 Rutherford (9) has developed an atomic model, in which the 

 positive charge resides on a nucleus at the centre of the atom, 

 this nucleus being itself small compared with the dimensions 

 of the atom as a whole. The Rutherford atom somewhat re- 

 sembles a planet surrounded by satellites. We can speak of 

 the nucleus and the " atmosphere " of electrons rotating 

 around it, as constituting the atom. In spite of its minute 

 size, the nucleus is mainly responsible for the mass of the 

 atom. An atom of hydrogen, for example, differs from an 

 atom of lead, in that the nucleus of hydrogen is very much 

 less massive than the nucleus of the lead atom, and at the same 

 time the number of electrons in the atmosphere of the hydro- 

 gen atom are very few compared with the number in the 

 atmosphere of the lead atom. We shall come to the question 

 of the numbers of satellite electrons in a moment. It is not 

 easy to enter into the reasoning which suggested this structure 

 to Rutherford. Suffice it to say, that the concept is based 

 essentially upon the behaviour of an atom when an alpha 

 particle from some radio-active source collides with it. Ruther- 

 ford has shown that an a particle — which is a positively 

 charged particle of matter whose mass is the same as that of 

 an atom of helium — passes with ease through the outer 

 " atmosphere " of electrons, which surround the centre of any 

 atom,| but tnat the a particle is violently deflected from its 



