TYPES OF VALENCE 391 



designate by the symbol v^- We shall then adopt the convention that the 

 electrovalence of an atom is positive when the atom gives up electrons and 

 negative when it takes up electrons. The electrovalence of an atom in any 

 compound may thus be defined as the number of electrons which the 

 neutral atom must give up in forming that compound. If the neutral atom 

 must take up electrons, the electrovalence is expressed as a negative num- 

 ber. The electrovalence of any atom is thus given by the expression 



Ve= e — s. (2) 



Electropositive atoms in complete compounds lose all the electrons in 

 their sheaths so that s is zero and therefore Ve is positive and equal to e. 

 For electronegative atoms i- is always greater than e so that Ve is negative. 



Let us define the covalence (vc) of an atom as the number of duplets 

 which that atom shares with neighboring atoms. Every duplet shared by 

 two atoms corresponds to a (covalence) bond between atoms, and we have 

 already represented the number of such bonds in a given group of atoms by 

 the S3'mbol B. If we now form 2(Vc) by adding the values ofvc for all the 

 atoms in the given group, we count each bond twice. Hence we may place 



^Ve=2B. (3) 



By substituting (3) and (2) in (i) and rearranging terms we find 



^Ve + 2z'c = O. (4) 



This simple result may be stated as follows : 



The sum of the electrovalences and covalences for all the atoms in any 

 complete compound is zero. 



Electrovalence and covalence are thus in a sense supplementary to one 

 another. If we represent Ve-\-Vc by v. Equation 4 takes the form 



^v = o (5) 



for any complete compound, and this suggests that the quantity v may have 

 some simple physical significance. 



In accordance with the nomenclature introduced by Lewis we may 

 define the kernel of an atom as that part of an atom which remains after 

 the sheath is removed. Since the neon atom has no sheath the whole atom 

 constitutes a kernel with zero charge. The kernel of the sodium atom is 

 the sodium ion with single positive charge, while the kernel of the fluorine 

 atom (or fluorine ion) consists of the nucleus and two electrons, the whole 

 having 7 positive charges. 



Since the sheath of any neutral atom consists of e electrons, the positive 

 charge on the kernel is also e. In any complete atom there are s electrons 

 in the sheath. When the atom does not share duplets with other atoms 



