Why Do Hydration Heats of Transition-Metal Ions Vary Irregularly with Atomic Number

Why Do Hydration Heats of Transition-Metal Ions Vary Irregularly with Atomic Number  

The theoretical discussion of the heats of ion-solvent interactions has been restricted so far to stressing the alkali metal and alkaline earth cations and halide anions. For these ions, a purely electrostatic theory (Section 2.15.10) provides fair coincidence with experiments. However, with the two- and three-valent transition-metal ions, where directed orbital interactions with water may have more influence. 

One way of seeing these changes is to plot the experimental heats of hydration of the transition-metal ions against their atomic number. It is seen that in the case of both divalent and trivalent ions, the heats of hydration lie on double-humped curves (Fig, 2.48). 

if the transition-metal ions had spherical charge distributions, then one would expect that with increasing atomic number toe would be a decreasing ionic radius and thus a smooth and monotonic increase of the heat of hydration as the atomic number increases. The double-humped curve implies therefore the operation 

In the case of transition-metal ions, the 3d orbitals are not spherically symmetrical in fact, they are as shown in Fig. 2.49. In a gaseous ion (i.e., a free unhydrated ion), all the 3d orbitals are equally likely to be occupied because they all correspond to the same energy. Now, consider what happens when the ion becomes hydrated by six water molecules situating themselves at the comers of an octahedron enveloping the ion. The lone electron pairs of the oxygen atoms (of the water molecules) exert a repulsive force on the valence electrons of the ion (Fig. 2.50). 

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