264 PHENOMENA, ATOMS, AND MOLECULES 



molecule is not a static field but is a pulsating or partly oscillating field in- 

 creasing in amplitude of oscillation very rapidly as the surface of the atom 

 is approached. Thus when an electron approaches an atom it acquires in 

 addition to any motion of translation it may have, an oscillatory motion due 

 to the pulsating field. Debye shows that this oscillatory motion tends to be 

 1 80° out of phase with the field producing it, and then shows, that because 

 of the non-uniformity of the electric field near an atom, the resulting force 

 will be one of repulsion. This repulsive force may be assumed to exist not 

 only for free electrons but for electrons which themselves are involved in 

 the orbital motions of other atoms. 



Thus the repulsive forces between atoms and molecules are due to the 

 perturbations in the electron orbits caused by the motions of the electrons 

 which revolve in orbits in neighboring atoms. The repulsive force does not 

 originate from an atom as a whole, but comes from the close approach of 

 electrons in the two atoms. Thus we should look upon these forces as 

 surface forces and should try to express them as functions of the distances 

 between the surfaces of the atoms (fixed by the outer electron orbits) 

 rather than in terms of the distances between the centers of the atoms. 



FORCES AT THE SURFACES OF MOLECULES 



In a discussion of the compressibility of metals and its relation to inter- 

 atomic forces ^ the writer has shown that the range of the attractive forces 

 between the atoms is approximately the same for all metals, although the 

 compressibilities of the metal and the volumes of the atoms vary over wide 

 ranges. Thus -although the compressibility of caesium is 220 times that of 

 tungsten, and the atomic volume is 7.4 times that of tungsten, the range of 

 the attractive force is nearly the same, 0.66 X 10"^ and 0.57 X 10"^ cm. 

 respectively. * 



Debye ^° in a paper of 1920 in analyzing the causes of attractive forces- 

 between non-polar molecules attempts to calculate the force from assumed 

 stationary electron configurations. For this purpose he expressed the 

 electric potential in the neighborhood of a given molecule in terms of an 

 infinite series expanded according to reciprocal powers of the distance r 

 from the center of the molecule. If the molecule is uncharged (i.e., is not 

 ionic) the first term which involves i/r vanishes. If the molecule is non- 

 polar (i.e., has no electric dipole moment) the second term involving i/r^ 

 vanishes. For a non-polar molecule, which is of the type of a quadrupole, 

 the third term is therefore the first term which does not vanish. Debye con- 

 siders therefore the effects due to this term involving i/r^ and neglects all 

 subsequent terms in the series. If, however, the molecule has a higher 

 degree of symmetry than that of a quadrupole (for example a tetrahedron 



