TYPES OF VALENCE 399 



elements and they thus have metallic properties, when in the elementary 

 form. For similar reasons all the known 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 valence 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 electropositive atoms can not be given up even when other atoms are 

 present that can take up electrons. It thus happens that the long periods 

 contain series of elements which all have 3 or 2 and 3 as their principal 

 valences. The atoms of these elements are therefore incomplete. The 

 electronegative atoms in such compounds, however, are always complete. 



It is of interest to note that as long as atoms are incomplete there seems 

 to be no tendency for them to have an even rather than an odd number of 

 electrons. For example, the following ions all have odd numbers of elec- 

 trons : Cv^'\ Mn"^ Fe*^% Co*\ and Cu^^ This seems to indicate that the 

 remarkable tendency, pointed out by Lewis, for most compounds to contain 

 even numbers of electrons is due merely to the relative abundance of com- 

 plete compounds as compared to incomplete ones. In other words, the even 

 number of electrons in most compounds results from the tendency of 

 Postulate I rather than from any more general tendency for electrons to 

 form pairs. 



Many of the compounds of this class, such as ZnO (zincite), Fe304, 

 PbS, CuO, etc., show electric conductivity even as solids. This is un- 

 doubtedly caused by the relatively large number of electrons in incomplete 

 sheaths. Of course we should not expect all compounds which contain such 

 electrons to show conductivity, for the presence of the electronegative 

 atoms might easily prevent the mobility of these electrons. We need to know 

 much more than we now do about the arrangement of the atoms and their 

 electrons in space before we can predict conductivity in particular cases 

 of this kind. 



3. Exceptional Cases. — There are some substances or compounds 

 whose structure is not adequately accounted for by the foregoing analysis. 

 A few examples are : N2, CO, CN", NO. The writer believes these have 

 tl;e single octet structure which he described in his earlier publications. It 

 is probable that acetylene, C2H2, and the carbide ion C2"" (in CaC2, etc.) 

 have the same kind of structure. Pease has suggested that they may all have 

 a triple bond structure.® This question merits careful study. 



^ Jour. Amer. Chem. Soc, 43, 991 (1921). 



