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ION–SOLVENT INTERACTIONS 41

1. Water is a very special solvent in respect to its structure (Section 2.4) and the
fact that nearly all of our knowledge of ions in solutions involves water arises from its
universal availability and the fact that most solutions met in practice are aqueous.
However, studying the hydration of ions rather than their solvation limits knowledge,
and a welcome modern trend is to study nonaqueous solutions as well.
2. Modern theoretical work on solution properties often involves the use of mean
spherical approximation, or MSA. This refers to models of events in solution in which
relatively simple properties are assumed for the real entities present so that the
mathematics can be solved analytically and the answer obtained in terms of an
analytical solution rather than from a computer program. Thus, it is assumed that the
ions concerned are spherical and incompressible. Reality is more complex than that
implied by the SE approximations, but they nevertheless provide a rapid way to obtain
experimentally consistent answers.

2.4. STRUCTURE OF THE MOST COMMON SOLVENT, WATER
One can start by examining the structure of water in its gaseous form. Water vapor
consists of separate water molecules. Each of these is a bent molecule, the H–O–H
angle being about 105° (Fig. 2.5). In the gaseous oxygen atom, there are six electrons
in the second shell (two 2s electrons and four 2p electrons). When the oxygen atoms
enter into bond formation with the hydrogen atoms of adjacent molecules (the liquid
phase), there is a blurring of the distinction between the s and p electrons. The six
electrons from oxygen and the two from hydrogen interact. It has been found that four
pairs of electrons tend to distribute themselves so that they are most likely to be found
in four approximately equivalent directions in space. Since the motion of electrons is
described by quantum mechanics, according to which one cannot specify precise orbits
for the electrons, one talks of the regions where the electrons are likely to be found as
orbitals, or blurred orbits. The electron orbitals in which the electron pairs are likely
to be found are arranged approximately along the directions joining the oxygen atom
to the corners of a tetrahedron (Fig. 2.6). The eight electrons around the oxygen are
neither s nor p electrons; they are hybrids. Of the four electron orbitals, two are
used for the O–H bond, and the remaining two are as free as a lone pair of electrons.

Fig. 2.5. A water molecule is nonlinear.