Physical Chemistry Third Edition

856 20 The Electronic States of Diatomic Molecules

Figure 20.16 The Orbital Region of the Bonding LiH Orbital.Positive regions are in

black and negative regions are in gray.

lithium nucleus if it occupies this space orbital. We call a chemical bond responding

to this situation apolar covalent bond. The ends of the molecule possess net charges,

giving the molecule adipole moment.

If a collection ofnelectrical charges has a net charge of zero, its dipole moment is

a vector defined by

μ

∑n

i 1

Qiri (20.4-9)

whereQiis the value of charge numberiandriis its position vector. If there are only

two electric charges+Qand−Qseparated by a distancer, the magnitude of its dipole

moment is

|μ|μQr (20.4-10)

The dipole moment vector points from the negative charge to the positive charge.

A larger dipole moment corresponds to a larger distance between the charges and/or

larger charges.

EXAMPLE20.10

For two charges of equal magnitude and opposite signs at arbitrary locations, show that the

magnitude of the vector in Eq. (20.4-9) is the same as that given by Eq. (20.4-10).