THE STRUCTURE OF ATOMS 33 



from the old but in each case in better agreement with the 

 facts. 



The theory indicates a series of new relationships be- 

 tween certain types of substances which I have termed 

 isosteric substances. For example, it indicates that the 

 molecules of carbon dioxide and nitrous oxide should 

 have nearly identical structures and this is borne out by 

 the extraordinary similarity in the physical properties 

 of these gases. Nitrogen and carbon monoxide constitute 

 another pair of gases which are similarly related. The 

 same theory also points out a number of previously un- 

 suspected cases of similarity of crystalline form (iso- 

 morphism). 



It is clear that in the past the term valence has been 

 used to cover what we may now recognize as three differ- 

 ent types of valence, as follows : 



I 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 which 

 an atom can share with its neighbors. 



It is recommended that only for valences of the co val- 

 ence type should definite bonds be indicated in chemical 

 formulas. One of the particular advantages of the pres- 

 ent theory is that it becomes easy to distinguish between 

 covalence and the other types and thus to predict with 

 certainty in what way electrolytic dissociation will occur 

 if at all. 



REFERENCES 



K. Kossel, Ann. Physik, 49 (1916), 229. 



G. N. Lewis, /. Am. Chem. Soc., 38 (1916), 762. 



Langmuir, /. Frank, hist., 187 (1919), 359; /. Am. Chem. Soc., 

 41 (1919), 868, 1543; 42 (1920), 274; Proc. Nat. Acad. Sci., 5 

 (1919), 252. 



