260 PHENOMENA, ATOMS, AND MOLECULES 



We know that molecules actually exist and must be of the most varied 

 shapes. Let us therefore consider how the various possible shapes of the 

 molecules and the types of force fields must influence the physical properties 

 of substances. 



TYPES OF INTERATOMIC AND INTERMOLECULAR FORCES 



The chemical properties of matter are primarily dependent on the 

 number of electrons in the atoms and the manner in which these electrons 

 are able to arrange themselves to form the most stable configurations. The 

 electric forces due to charges on the atoms frequently oppose the formation 

 of certain configurations which, except for these charges, might be the most 

 stable. The effect of these two factors in governing chemical action has been 

 discussed at some length by the writer.f 



The electron configurations existing in the atoms of the inert gases He, 

 Ne, Ar, etc., are the most stable of all. Other atoms, by taking up, or 

 giving up electrons, or by sharing electrons with each other, strive to obtain 

 electron configurations like those of the inert gases. It is mainly as a result 

 of these changes that chemical interaction occurs. 



Each of the ions O"", F", Na"", and Mg^* contains lo electrons, the same 

 number as in a neon atom. The reason for the formation of chemical com- 

 pounds containing these ions is that the electrons can thereby become 

 arranged in the stable configuration characteristic of the neon atom. 



This view, which is largely that of Lewis and Kossel, and is now 

 generally accepted, may serve as a basis for a classification of inter- 

 molecular forces. There are three main types of molecules to consider : * 

 (i) Non-polar, (2) Polar, and (3) Ionic, as follows: ^ - 



I. Non-polar. 



The neon atom, or methane molecule, may be taken as the most extreme 

 example. The center of gravity of the electrons of the molecule coincides 

 with that of the positive charges of the nuclei, so that there is no dipole 

 moment. In general, however, the atom or molecule may behave as an 

 electric quadrupole or system or even higher symmetry. Debye has shown 

 that quadrupole molecules f attract one another, when at a considerable 



t "Types of Valence," I. Langmuir, Science, 54, 59-67 (1921). The recent work 

 of Bohr, which has proved that the electrons in the heavy atoms (such as nitron) are^ 

 arranged in shells according to the scheme 2, 8, 18, 32, 18, 8 instead of the scheme 2,* 

 8, 8, 18, 18, 32 postulated by the writer, requires only a slight and obvious modifica- 

 tion of the wording of Postulate I, and does not otherwise affect the validity of the 

 conclusions that were drawn in that paper. 



*P. Debye, Phys. Zeit., 21, 178 (1920), has made this classification based on the 

 degree of symmetry of the distribution of electrons within atoms. 



t The word molecule will be taken to include atoms in those cases where the 

 atoms act as individuals, as, for example, in gaseous or liquid neon. 



