POPULAR SCIENCE 627 



tricity in the atom. The first successful attempt to deal with 

 the structure of the atom from the electronic standpoint is 

 that of Sir J. J. Thomson (8), who postulated a sphere of posi- 

 tive electricity of uniform density throughout which the elec- 

 trons were distributed. This was frankly an attempt to give 

 stability to the atom rather than a final statement of the 

 nature and location of the positive charge. Stress was laid 

 on the problem of distribution of electrons in such an atom, 

 especially with the view of showing how certain chemical pro- 

 perties possessed by actual atoms — notably the periodicity 

 in properties as expressed in the Periodic Law — could be ac- 

 counted for in terms of electrons. The problem is to find how 

 the electrons will distribute themselves in a sphere of positive 

 electricity, when successive additions of electrons are imagined 

 to be made to the system. To simplify the problem, Sir J. J. 

 Thomson considered the electron distribution in one plane, 

 namely, the plane through the centre of the atom. The results 

 were by no means complete, but they showed in a striking 

 manner that the first step towards the elucidation of atomic 

 structure had been taken. We shall follow out a few of the 

 simpler steps in this imaginary process of building up an atom 

 model. 



If one electron be added to the positive sphere it will 

 obviously go to the centre and remain. If a second electron 

 be added, the two will take up positions equidistant from the 

 centre, their distance apart being identical with the radius of 

 the sphere. Three electrons will distribute themselves at the 

 apices of an equilateral triangle. Four electrons were shown 

 to be incapable of stable equilibrium in one plane. With five 

 electrons the equilibrium state reached corresponds to a 

 regular pentagon, one electron being at each corner. Six 

 electrons, however, would not form a stable hexagonal system. 

 Instead, one electron goes to the centre, the remaining five 

 forming a regular pentagon. This is called a two-ring system. 

 Similarly eleven electrons distribute themselves in such a way 

 that two form the inner ring, and nine the outer. With suc- 

 cessive additions one finds that the two-ring system becomes 

 unstable, and a three-ring system makes its appearance. This 

 occurs when seventeen electrons are present, and the three- 

 ring system persists until we reach thirty-two electrons, when 

 a four-ring system appears. In this way we can build up 



