310 PHENOMENA, ATOMS, AND MOLECULES 



By the aid of the viewpoint developed above, let us analyze more closely 

 the probable mechanism of condensation, reflection and evaporation of 

 atoms or molecules. 



If an atom (or molecule) A approaches the surface of a solid body, it 

 will be subjected to an attractive force as soon as it comes within a certain 

 distance of the surface. This force will increase in intensity as A ap- 

 proaches nearer the surface, will pass through a maximum and will then 

 decrease to zero when A reaches the equilibrium position. The work done 

 by the attractive forces will have been converted into kinetic energy, part 

 of which is possessed by the atom and the remainder of which is divided 

 among atoms forming the surface. 



The kinetic energy of A will carry it past the equilibrium position into 

 a region in which it is subjected to a repulsive force. The component of 

 its velocity toward the surface will thus decrease to zero. It will then be 

 driven back through the equilibrium position, and if its outward velocity 

 is sufficient it may escape from the region of attractive forces and travel 

 indefinitely away from the surface. On the other hand, if its velocity is 

 not great enough to enable it to escape, it will describe a complex orbit 

 and, by loss of energy to adjacent atoms, will finally oscillate about its 

 equilibrium position with a mean kinetic energy corresponding to the tem- 

 perature of the surface. 



The amount of work done by the attractive forces upon an atom A 

 while it moves to an equilibrium position is measured by the heat of 

 evaporation or adsorption and is equal to l/N, where X is the internal heat 

 of evaporation per gram molecule and N is equal to 6.062 X 10^^. In the 

 case of adsorption of a gas by a solid X represents the heat of adsorption. 



If the attractive and the repulsive forces were both exerted by a single 

 atom of the solid, and if this atom could move freely towards the incident 

 atom, then we should be justified in treating the problem as if attractive 

 forces were absent and elastic collisions occurred. In this case we could 

 apply Baule's results directly. 



But it is almost certain that the attractive forces are exerted by several 

 atoms. Bragg has found that among metals the face centered cubic lattice 

 structure is the most common. In this lattice each atom is surrounded by 

 twelve others equally spaced from it. An atom on the surface must then 

 usually have from four to eight other atoms exerting force on it. On the 

 other hand, it is very probable that an atom which passes through the 

 equilibrium position with high velocity would be subjected to repulsive 

 forces exerted principally by ojie atom of the solid. This would naturally 

 follow from the fact that the repulsive force must increase with the 

 proximity to an atom much more rapidly than the attractive force decreases 

 on receding from the equilibrium position. 



