chapter Eighteen 

 TYPES OF VALENCE 



From the theory proposed by G. N. Lewis in 1916 and subsequently 

 extended by the writer, it is clear ^ that the term valence has been used in 

 the past to cover what we may now recognize as three distinct types of 

 valence, viz. : 



1. Positive valence: the number of electrons an atom can give up. 



2. Negative valence : the number of electrons an atom can take up. 



3. Covalence : the number of pairs of electrons an atom can share with 

 its neighbors. 



It was shown that these fundamental conceptions of valence as well as 

 the actual numerical values of each type of valence for most of the elements 

 could be derived from a few postulates regarding the structure of atoms. 

 The following method of deriving these relationships, however, is not only 

 much simpler than that previously given by the writer, but throws a new 

 light on the relationships between the different types of valence. 



We will take for granted the Rutherford type of atom, which consists of 

 a positive nucleus surrounded by a number of electrons equal to the atomic 

 number of the atom. We will also assume that Coulomb's law applies to 

 the forces between the charged particles in the atom, but at the same time 

 will recognize the existence of repulsive forces in atoms which prevent the 

 electrons from falling into the nucleus. For the present purpose, however, it 

 is immaterial whether the repulsive force is a dynamic force (centrifugal 

 force) such as that assumed by Bohr, or is a static force as postulated by 

 G. N. Lewis, J. J. Thomson or recently by the writer.^ 



We shall need to make only 3 postulates in regard to the structure of 

 atoms, and these are consistent with those previously proposed. 



Postulate I. — The electrons in atoms tend to surround the nucleus in 

 successive layers containing 2, 8, 8, 18, 18 and t,2 electrons respectively. 



The word atom is used in the broader sense which includes charged 

 atoms (ions). If the number of electrons in an atom is such that they can 

 not all form into complete layers in accord with Postulate i, the extra 



^ I. Langmuir, Jour. Amer. Chem. Soc, 41, 926 (1919), and Jour. Ind. Eng. 

 Chem., 12, 386 (1920). 



^ Science, 53, 290, Mar. 25, 1921. 



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