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42 CHAPTER 2

Fig. 2.6. The hybrid orbitals
of an oxygen atom.

Because of the repulsion of the electron pairs, the H–O–H angle is not exactly equal
to the tetrahedral angle but is a few degrees less than that.
Water molecules vibrate and rotate in the gas phase. In the liquid phase, the
rotation is hindered (libration) because of the structure (intermolecular H bonds) and
the vibrations are modified (Section 2.11.2).
The free orbitals in which are found the electron lone pairs confer an interesting
property on the water molecule. The center of gravity (Fig. 2.7) of the negative charge
in the water molecule does not coincide with the center of gravity of the positive
charge. In other words, there is a separation within the electrically neutral water
molecule: it is thus called an electric dipole. The moment of a dipole is the product of
the electrical charge at either end times the distance between the centers of the electrical
charge, The dipole moment of water is 1.87 D in the gas phase (but becomes
larger when the water molecule is associated with other water molecules) (Section
2.4.2).
In fact, although water can be treated effectively as a dipole (two equal and
opposite charges at either end of a straight line), a more accurate representation of the
electrical aspects of water is to regard the oxygen atom as having two charges and each
hydrogen atom as having one. This model will be studied in the theory of hydration
heats (Section 2.15).

Fig. 2.7. A water molecule can be considered
electrically equivalent to a dipole.