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ION–SOLVENT INTERACTIONS 43

Fig. 2.8. The oxygen at-
oms in ice, which are lo-
cated at the intersections of
the lines in the diagram, lie
in a network of open puck-
ered hexagonal rings.

The availability of the free orbitals (with lone electron pairs) on the oxygen atom
contributes not only to the dipolar character of the water molecule but also to another
interesting consequence. The two lone pairs can be used for electrostatic bonding to
two other hydrogen atoms from a neighboring water molecule. This is what happens
in a crystal of ice. The oxygen atoms lie in layers, with each layer consisting of a
network of open, puckered hexagonal rings (Fig. 2.8). Each oxygen atom is tetrahe-
drally surrounded by four other oxygen atoms. In between any two oxygen atoms is a
hydrogen atom (Fig. 2.9), which provides a hydrogen bond. At any instant the
hydrogen atoms are not situated exactly halfway between two oxygens. Each oxygen
has two hydrogen atoms near it (the two hydrogen atoms of the water molecule) at an
estimated distance of about 175 pm. Such a network of water molecules contains
interstitial regions (between the tetrahedra) that are larger than the dimensions of a
water molecule (Fig. 2.10). Hence, a free, nonassociated water molecule can enter the
interstitial regions with little disruption of the network structure.
This important property of water, its tendency to form the so-called H bonds with
certain other atoms, is the origin of its special characteristic, the netting up of many
water molecules to form large groups (Fig. 2.10). H bonds may involve other types of

Fig. 2.9. The hydrogen
bond between two oxygen
atoms (the oxygen and hy-
drogen atoms are indicated
by and respectively).