398 PHENOMENA, ATOMS, AND MOLECULES 



2. Incomplete Compounds. — These are compounds in which some of 

 the electrons are not arranged in complete layers or sheaths, so that the 

 tendency of Postulate i is not completely satisfied. This can only occur as 

 a result of a conflict between Postulate i and Coulomb's law or Postulate 3. 

 We have seen that the tendency of Postulate 3 causes the residual charge 

 {v) on each atom to be a minimum. The tendency of Postulate i, however. 

 is sufficiently strong to force the atoms to take up charges of 3, 4, or even 

 under some conditions, 5 or 6 units, if this should be necessary in order to 

 bring all the electrons into complete layers. Since there must be a limit to 

 the strength of the tendency of Postulate i it is not surprising that residual 

 atomic charges greater than 4 or 6 are very rare. Now the atoms of the 

 elements near the middles of the long periods (of 18 and 32 elements), do 

 not become complete even if they do acquire residual charges as great as 5 

 or 6 units, and it is therefore natural that the tendency of Postulate 3, 

 which must become stronger as the charge increases, should prevent the 

 formation of complete compounds of these elements. There are two types 

 of incomplete compounds to consider. 



a. Metallic Substances. Electronegative Atoms Absent. — By Coulomb's 

 law, atoms having only small charges on their kernels, should not be able 

 to take up enough electrons to complete sheaths of 8 or more electrons. 

 Thus if we bring together a number of electropositive atoms there is no 

 way in which the electrons in the incomplete sheaths can rearrange them- 

 selves to form complete sheaths. The "free" electrons which are thus 

 compelled to remain in incomplete sheaths are responsible for the metallic 

 properties shown by all electropositive elements in the solid or liquid state. 

 It is clear, however, notwithstanding the fact that hydrogen may some- 

 times function as an electropositive element, that liquid or solid hydrogen 

 should have none of these metallic properties according to this theory, for 

 the sheath to be formed in this case contains only two electrons. The forces 

 acting between the free electrons and the kernels of the atoms in metallic 

 substances, are of the same order of magnitude as in salts, so that metals 

 have about the same range of vapor pressures, hardness, compressibilities, 

 etc., that are shown by salts. 



In general, all atoms must be electropositive unless they can take up 

 enough electrons to complete their sheaths and thus act as electronegative 

 atoms. The tendency of Postulate 3 ordinarily prevents the occurrence of 

 negative valences greater than about 4. In the two short periods eight 

 electrons are needed to form a complete sheath so that the elements with 

 kernel charges greater than about 3 can act as electronegative atoms and 

 therefore do not normally show metallic properties. In the 2 long periods 

 18 electrons form the complete sheath so that about the first 14 of the 

 elements in each of these periods can usually act only as electropositive 



