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212 CHAPTER 2

and therefore by substitution of Eq. (A2.3.17) in Eq. (A2.3.6),

The first term on the right-hand side of Eq. (A2.3.18) can be rearranged as follows:

Thus, as a first approximation, the water molecule can be represented as a dipolar
charge distribution in which there is a positive charge of –2q (due to the H atoms) at
a distance from the origin on the bisector of the H–O–H angle and a charge of –2q
(due to the lone electron pair) at a distance from the origin; it follows that

magnitude of each charge of dipole

× distance of the charge from origin (A2.3.20)

The right-hand side of this expression is the general expression for the dipole moment
as is seen by considering the situation when i.e., where 2d
is the distance between the charges of the dipole, in which case one obtains the familiar
expression for the dipole moment

Thus, the first term on the right-hand side of Eq. (A2.3.18) is

The second term can be interpreted as follows: Consider It can be
written thus