40 



PHENOMENA, ATOMS, AND MOLECULES 



which form the surface of the Hquid (per sq. cm.) in excess of that which 

 the same molecules possess when they are in the interior of the liquid. 



In Table I data are given for a few substances which serve to illustrate 

 the interrelation (19) (20) between the orientations of molecules and other 

 properties of liquids. The latent heat of evaporation, X, which, according 

 to Trouton's Rule, is approximately proportional to the absolute boiling 

 point, Tb, serves as a measure of the energy needed to bring a molecule 

 from the interior of the liquid out into the vapor phase. We have seen that 

 the surface energy yo is the energy per unit area needed to form a surface. 



The molecular volume V given in this table is the volume of the liquid 

 divided by the number of molecules. The molecular surface S is the surface 

 of a sphere which has the volume V. The extremely low values of "k, the 

 heat of evaporation, per molecule, for helium and hydrogen, indicate that 

 the forces exerted by these molecules on their neighbours are unusually 

 weak. 



TABLE I 



Surface Properties of Molecules 



Substance 



Unit = 



V 



Volume 



A" 



S 



Surface 



A* 



Boiling 

 Point 



I 



Heat of 



Evaporation 

 Erg X 10-^* 



;./s 



Erg cm— ^ 



Erg cm- 



He .. . 



H - H . . 



H - OH . 



A . . . . 



CH, . . . 



n-QH,;OH 



0.24 



1.67 



67. 



II. 3 



16.3 



56. 



82. 



0.35 



2.7 

 140. 

 18. 

 21. 

 28. 

 41.5 



0.59 



5-4 

 118. 



35-3 



50.7 

 50.7 



This is in accord with the great stabihty of the electron pair which forms 

 the K shell of atoms and the covalence bond. The diameters of the helium 

 atom and hydrogen molecule given by the kinetic theory from viscosity 

 measurements, viz., 1.9 and 2.4 A, correspond to volumes V of only 3.6 and 

 7.2 A^, which are very small compared to the volumes occupied in the 

 liquid. This very open structure of the liquid is again an indication of the 

 weak attractive forces between these molecules. 



The effect of replacing one of the hydrogen atoms in the hydrogen 

 molecule by the hydroxyl radical, thus producing water, is to decrease the 

 value of V from 47. to 30., an indication of strong forces acting between 

 the hydroxyl groups which compress the liquid. A better measure of these 



