46 ILLINOIS STATE ACADEMY OF SCIENCE 



should be at a loss whether to put one dash to represent 

 the sodium atom, or six. With the understanding, how- 

 ever, which the physicist has given us of the structure of 

 sodium chloride, the uncertainty of th^e valence number 

 causes us no difficulty; rather, the new viewpoint of the 

 matter makes easily understandable the many interest- 

 ing properties of that large class of substances of which 

 sodium is a conspicuous example, known as electrolytes. 



On the other hand, in the case of the organic com- 

 pounds, where the number system of valence works so 

 satisfactorily, we have no physical evidence as to the 

 structure or arrangement of the atoms in the molecules 

 except that the atoms do not seem to be charged as they 

 are in the case of sodium chloride. We must still rely 

 upon the intuition of the chemist rather than the direct 

 experiment of the physicist for an explanation of the 

 force which we call a valence bond. Many chemists still 

 prefer to regard valence as an abstract numerical prop- 

 erty of the atom and content themselves with taking it 

 as it exists without worrying about why it is so. On the 

 other hand, with the conception of the atom as a positive 

 nucleus whose charge is its atomic number surrounded 

 by electrons, a physical chemist believes that valence is 

 not simply a number, but that it can be explained in all 

 cases as due to electrostatic forces between the positive 

 nucleus of one atom, and the outer electrons of the same 

 and other atoms. The theory of G. N. Lewis has met 

 with most remarkable success in explaining the combina- 

 tion of atoms in the molecules. This theory is based 

 upon the conception of the atom which the physicist has 

 given us, and I shall try to sketch for you its essential 

 features. 



Let us consider the elements from lithium to neon in- 

 clusive, which form the first row of eight in the periodic 

 table. Lithium has an atomic number of three, glucinum, 

 four, boron five, and so on, to neon with an atomic num- 

 ber of ten, the elements being placed according to their 

 atomic number. The atomic number is the number of 

 positive charges on the nucleus, and we represent the 

 nucleus of the atom by writing the atomic number with 



