876 Some Dimensions of the Atom. 



on opposite sides o£ the positive nucleus, and we can obviously 

 represent the atom by a plane diagram. 



E<? 



Attractive force on £ x 



(1-15) 2 ' 



Repulsive force on e l = — 



(2«30) 2 ' 



e e 



there! ore -— — — = - and so E = *25<?. 



(l-lo) 2 (2-30) 2 



These values for E are calculated on the presumption 

 that (a) the electrons are not in motion round the positive 

 nucleus, and so have no acceleration towards the centre, and 

 (b) only electrostatic forces are present. It may be mentioned 

 here, however, that in the case of atoms where two electrons 

 are squeezed into the space nominally provided for one, then 

 there is a second force which comes into play, and this- 

 exactly balances the repulsive electrostatic force between the 

 electrons. It is therefore an attractive force of some nature.. 



If we do not presume this, then when we meet with the 

 first case of two electrons above one another there will be a 

 very large increase in the value of E, which although 

 possible is not probable. We can, however, find the distance 

 to which these two forces annul each other. 



Let us take a completed Neon atom, then upon adding 

 eight more electrons we shall obtain an Argon atom, of 

 course the positive nucleus must alter at the same time. All 

 the shells I, II, and II b are now completed, and in the 

 second shell we have crowded two electrons into the space 

 for one. 



If one above the other, it seems feasible that the distance 

 between them should be the same as the distance between 

 the radii of the shells II a and II6 = *14x 10~ 8 cm. Con- 

 sequently at this distance the two forces balance; but this 

 attractive force does not seem to be noticeable between 

 electrons which are a greater distance apart. Therefore it 

 probably varies as a higher power of the distance than 

 the 2nd. 



Owing to the complexity of the elements with atomic 

 numbers above 10/it becomes very difficult to calculate the 

 dimensions of E. From the value of b in Van der Waals' 

 equation we obtain no information respecting the actual 

 arrangement of the electrons in the atom. Langmuir's 

 positions for them have been used throughout; the figures 

 only showing that when moving from one inert gas to 

 the next the radius of the atomic sphere increases by a 

 constant quantity. 



