TYPES OF VALENCE 395 



If, however, we place Vc = Vn for each atom it is evident that Equation 6 

 will be satisfied. The residual charge on every atom (being — Vn-{-Vo) is 

 then zero. Thus in any group of atoms Postulates i and j are both com- 

 pletely satisfied if the covalence of each atom is equal to the negative 

 valence of that atom. The negative valence of carbon, nitrogen, oxygen 

 and sulfur are 4, 3, 2 and 2 respectively, while that of hydrogen and the 

 halogens is one. If therefore we follow the custom of the organic chemist 

 and write structural formulas using these valences we obtain results in com- 

 plete accord with Postulates i, 2 and 3. 



Thus these 3 postulates lead us to a rational derivation of the empirical 

 valence rules which constitute the foundation of the science of organic 

 chemistry. Moreover we are brought to see clearly the limitations of this 

 empirical theory. We now realize that it is only negative valences that 

 should be used in structural formulas {i.e., as covalences) and that even 

 these can only legitimately be used in compounds in which electropositive 

 atoms are entirely absent, for if some of the atoms have a positive residual 

 charge (v = Ve) then from Eq. 5 it is evident that other atoms must have 

 a negative charge, and for these as well as the electropositive atoms the 

 covalence is not equal to the negative valence. 



From this viewpoint it is incorrect to write structural formulas such 

 as Na — CI, 



H— O O 



V 



-/\ 



H 



etc., in which the covalence of one atom is taken as equal to the positive 

 valence of that atom. 



It should be kept in mind that Postulate 3 does not require that v^ 

 should be equal to Vn. There is merely a tendency for these valences to be 

 equal. Among compounds of electropositive elements we saw that there 

 was a conflict between the tendencies of Postulates i and 3 so that v was 

 always different from zero. With compounds formed exclusively of electro- 

 negative atoms, however, there is not necessarily a conflict and it is for 

 this reason that we have such a large class of compounds in which v is zero 

 {i.e., Vc = Vn). There may be various causes that make it difficult for v 

 to be zero even for some compounds of electronegative elements, so that in 

 individual cases v may differ from zero by one or two units. It must be 

 remembered that we deduced the relation v = minimum from Postulate 3 

 only by assuming the two atoms which share a duplet are of substantially 

 the same size, etc. From Coulomb's law we should expect that either a large 

 charge on the kernel of an atom or a small radius for the kernel should 



