SURFACE CHEMISTRY 61 



(3) Valence forces, associated with the sharing of electrons between 

 atoms. 



(4) Van der Waals' attractive forces, which depend on the mutual 

 polarizability of molecules. These forces are usually considered to vary 

 with l/r^. 



(5) Repulsive forces due to the mutual impenetrability of completed 

 electronic shells. It is primarily these which determine the collision areas 

 of molecules in the kinetic theory. Born and Mayer (33) find that the 

 potential energy of these repulsive forces between two completed shells is 

 given by 



A exp [(ri + Ti — r)/ro] , 



where ri and r2 are the effective radii of the two shells and ro is a universal 

 constant equal to 0.435 ^> while A is another universal constant. This 

 means that this type of repulsive force between two atoms can be expressed 

 as a function of the distances between the surfaces of the atoms, this 

 function being the same for nearly all atoms (or molecules) which have 

 completed shells, regardless of their electric charge. From a consideration 

 of several of the other properties of solids and liquids I had previously 

 drawn a similar conclusion (Ref. 20, pp. 529-531), that the repulsive 

 forces should be regarded «as surface forces which should be expressed as 

 functions of the distances between the surfaces of the atoms (fixed by the 

 electron orbits), rather than in terms of the distances between the centers 

 of the atoms». A consideration of van der Waals' forces showed that for 

 these also the «intensities of the fields around different non-polar molecules 

 are practically identical when regarded as surface forces*. 



The fact that these repulsive forces, according to Born and Mayer, 

 decrease to 1/e*^ value for each increment of o.435A° in the distance 

 between the atoms, indicates that they have a very small range of action. 



(6) Electron pressures. The force that counterbalances the Coulomb 

 attraction of the electrons and ions in the alkali metals — and probably in 

 other metals — is that due to the pressure of the Fermi electron gas. 



Since all of these forces can be effective at the surface as well as in the 

 interiors of solids and liquids, they must take part in adsorption phe- 

 nomena. There should thus be various types of adsorption corresponding 

 to the several types of forces which hold the atoms or molecules on the 

 surface. Let us consider some examples : 



Caesium on Tungsten. The caesium ions are held by Coulomb attrac- 

 tive forces (here a modified image force) to the underlying metal. The 

 corresponding repulsive forces which hold the ion a definite distance from 

 the surface (and thus determine the dipole moment M) result from the 

 pressure gradient in the Fermi electron gas which extends beyond the 



