the Molecule and Chemical Combination. 511 



of a positive charge and, except for hydrogen, a considerably 

 larger number of electrons, we have to consider what happens 

 when two or more such atoms are brought close together ; 

 what kind of rearrangement of the electrons takes place when 

 the atoms unite and form a coherent system, and what are the 

 considerations which determine the properties and stability 

 of such a system. 



I assume, as in my paper in the Phil. Mag. April 1919, 

 that in the atom the electrons are in equilibrium under their 

 mutual repulsions and the attractions exerted upon them by 

 the positive charges. The repulsions between the electrons 

 are supposed to follow the usual law and to vary inversely as 

 the square of the distance. The force between the positive 

 charge and the atom is supposed to be more complicated, 

 and to vary with the distance r according to a law ex- 

 pressed by (f> I ) where c is a length. When r is either very 

 large or very small compared with c, <f>[) reduces to 1/r 2 ' 



but when r is comparable with c the force is no longer of this 

 simple type, but vanishes at certain distances, changing from 

 attraction to repulsion or vice versa. 



My reasons for preferring this equilibrium arrangement to 

 the more usual one of electrons describing orbits under forces 

 varying rigorously as the inverse square, is that unless each 

 electron is allowed to have a separate and isolated orbit the 

 orbital arrangement is essentially unstable. Thus if the two 

 electrons in a helium atom were to follow one another round 

 the same circular orbit, the system would be quite unstable. 

 Again, the scattering of .light by hydrogen molecules is not 

 that which would occur if the electrons in the molecule 

 described orbits (J. J. Thomson, Phil. Mag. ser. 6, vol. xl. 

 p. 398). 



When we have to consider molecules containing perhaps 

 dozens of electrons, the motion would be so hopelessly intricate 

 and confused if these were all describing these large orbits 

 that, apart from the fundamental difficulty about stability, 

 the model would give us but little assistance in forming a 

 mental picture of what goes on in chemical combination. 



Assuming that the positive charge exerts on an electron a 

 central force which changes from attraction to repulsion and 

 therefore vanishes at certain distances, we proceed to consider 

 how the electrons would be arranged round a central charge. 



If there is only one electron in the atom it must be at one 

 of the places where the force exerted by the positive charge 

 vanishes, and in order that the equilibrium may be stable the 



