80 ELECTRICAL CONDUCTIVITIES, ETC. 



HYDRATION AND IONIC VOLUME. 



While discussing the hydrating powers of different ions, the following relation 

 should be pointed out. Jones and Pearce,* after calling attention to the fact that the 

 hj'drating power of any salt is primarily a function of the cation, point out this relation : 



If the atomic volumes of the elements are plotted as ordinates against the atomic 

 weights as abscissas, we have the well-known atomic-volume curve. The curve con- 

 tains well-defined maxima and minima. The alkali metals fall at the maxima of the 

 curve. The three elements with the largest atomic volumes are potassium, rubid- 

 ium, and caesium. Salts of these metals usually crystallize from aqueous solution 

 without water of crystallization, and they, therefore, have very little hydrating 

 power. Lithium and sodium, some of whose salts crystallize with two and three 

 molecules of water, and which, therefore, show some hydrating power in solution, 

 have much smaller atomic volumes. At the minimum of the third section of the 

 atomic-volume curve we find the elements strontium, iron, cobalt, copper, and 

 nickel. The salts of these metals crystallize with relatively large amounts of water, 

 and they show great hydrating power in solution. Aluminium, which has less than 

 half the atomic weight of iron, but slightly greater atomic volume, falls at the second 

 minimum of the atomic-volume curve. Its salts crystallize with six and eight 

 molecules of water and show great hydrating power in solution. 



Comparing the metals of the calcium group, we find that barium, whose salts crys- 

 tallize with two molecules of water, has the largest atomic volume. The salts of the 

 other elements of this group crystallize each with six molecules of water, with the 

 exception of calcium nitrate, which crystallizes with four molecules. The magne- 

 sium ion, which has the smallest atomic volume of any element of this group, has the 

 greatest hydrating power. Strontium, which has a slightly larger atomic volume 

 than calcium, has a somewhat smaller hydrating power than calcium. 



A careful examination of all of the evidence available shows that the hydrating 

 power of the cation is an inverse function of its atomic volume. 



This explains why it is that ions with large mass often have larger migration veloc- 

 ities than ions with smaller mass, which is the reverse of what would be expected. 

 Thus, potassium, rubidium, and caesium have larger migration velocities than sodium 

 and lithium, notwithstanding the greater mass and volume of the former. This 

 was for a long time inexplicable. We now have the explanation. Lithium and 

 sodium have smaller atomic volume than potassium, rubidium, and caesium, and, 

 consequently, greater hydrating power. The hydrated lithium and sodium ions 

 move more slowly, due to the atmosphere of the solvent which they must drag with 

 them through the solution. 



A large number of similar relations have been pointed out by Jones and Pearce. | 



The question arises, Why this relation between hydrating power and atomic vol- 

 ume? It probably has to do with the electrical density upon the ion. The smaller 

 the ion the greater the electrical density, and, consequently, the greater the power 

 of the ion to condense molecules of the solvent upon it and hold them there in a state 

 of loose combination. 



*Amer. Chem. Journ., 38, 736 (1907). flbid, 38, 737-740 (1907). 



