chapter Eleven 



ISOMORPHISM, ISOSTERISM AND 

 COVALENCE 



I HAVE DESCRIBED a theory of valence which I have called the octet theory.^ 

 This theory is based upon and is essentially an extension of G. N. Lewis' 

 theory of the "cubical atom." - According to this theory each bond between 

 adjacent atoms in organic compounds corresponds to a pair of electrons 

 held in common by the two atoms. Since in other types of compounds the 

 number of pairs of electrons held in common is not always the same as the 

 number of valence bonds that have usually been assumed I proposed that 

 the number of pairs of electrons which any given atom shares with the 

 adjacent atoms be called the covalence of that atom. It was then shown 

 that the covalence of carbon is always 4, that of nitrogen is usually 3 or 4, 

 while that of oxygen is 1,2 or sometimes 3, etc. 



The octet theory indicates that the number and arrangement of electrons 

 in the nitrogen molecule, the carbon monoxide molecule, and the cyanogen 

 ion, is the same, and it was shown in fact that the physical properties of 

 nitrogen and carbon monoxide are remarkably alike. 



A similar relationship was found to exist between nitrous oxide and 

 carbon dioxide and again between cyanic and hydronitric acid. 



The following data, taken from Landolt-Bornstein's tables and Abegg's 

 handbook, show the extraordinary agreement between physical properties 

 of carbon dioxide and nitrous oxide: 



Both gases form hydrates, N2O.6H2O and CO2.6H2O. The vapor 

 pressure of the hydrate of nitrous oxide is 5 atm. at —6°, whereas the 

 hydrate of carbon dioxide has this vapor pressure at —9°. The heats of 

 formation of the two hydrates are given, respectively, as 14900 and 15000 

 calories per mol. The surface tension of liquid nitrous oxide is 2.9 dynes 

 per cm. at 12.2°, while carbon dioxide has this same surface tension at 9.0°. 



' A simpler exposition of the theory and its applications is given by Elwood 

 Hendrick, Met. Chcm. Eng., 21, 72, (1919), July 15th; Langmuir, Jour. Frank. Inst., 

 ^^7, 359 (1919) ; Jour. Amcr. Chcm. .Soc, 41. 868-934 (1919) ; Proc. Nat. Acad. Sci., 

 3, 252 (1919)- 



^ G. N. Lewis, Jour, .liner. Chcm. Sac, 38, 762 (1916). 



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