66 



SCIENCE 



[N. S. Vol. LIV. No. 1386 



the crystalline state and their molecular 

 weights are unknown. 



Molybdenum earbonyl, Mo(CO)g,-'' is a very 

 easily volatile crystalline compound. It is 

 interesting to note that the negative valence 

 of molybdenum (s — e ^ 18 — 6) is twelve, so 

 that with a covalence of 12 for the molyb- 

 denum atom in this compound we again ob- 

 tain a structure consistent with the valence 

 theory discussed above. 



2. Incomplete Compounds. — These are com- 

 pounds in which some of the electrons are not 

 arranged in complete layers or sheaths, so 

 that the tendency of Postulate 1 is not com- 

 pletely satisfied. This can only occur as a 

 result of a conflict between Postulate 1 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 1, how- 

 ever, 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 Postu- 

 late 1 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 forma- 

 tion 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 hav- 

 ing 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 electroposi- 

 tive atoms there is no way in which the elec- 

 trons in the incomplete sheaths can rearrange 

 themselves to form complete sheaths. The 

 sMond, Hirtz, Cowap, J. Chem. Soc, 97, 798 

 (1910). 



" free " electrons which are thus compelled to 

 remain in incomplete sheaths are responsible 

 for the metallic properties shown by all electro- 

 positive elements in the solid or liquid state. 

 It is clear, however, notwithstanding the fact 

 that hydrogen may sometimes 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, hard- 

 ness, compressibilities, etc., that are shown 

 by salts. 



In general, all atoms must be electroposi- 

 tive unless they can take up enough electrons 

 to complete their sheaths and thus act as elec- 

 tronegative atoms. The tendency of Postulate 

 3 ordinarily prevents the occurrence of nega- 

 tive 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 ele- 

 ments in each of these periods can usually act 

 only as electropositive elements and they thus 

 have metallic properties, when in the element- 

 ary form. For similar reasons all the knovm 

 elements of the rare earth period (the last 

 two being unknown) have metallic properties. 



h. Compounds Containing Electropositive 

 and Electronegative Atoms. — As a result of 

 Coulomb's law or Postulate 3, the positive va- 

 lence of an element is usually limited to a 

 value of 2 or 3 unless particularly strong 

 forces are exerted to draw away electrons, 

 and thus raise the positive valence a few units 

 higher. Thus in the middle of the long periods 

 the charges of the kernels are so great that 

 all the electrons in the sheaths of the electro- 

 positive atoms can not be given up even when 

 other atoms are present that can take up 

 electrons. It thus happens that the long pe- 



