Permanent Dipoles

Molecules which have an uneven distribution of charge (one end more positive than the other) are polar. Polar molecules are said to be permanent dipoles and have a permanent dipole moment () These molecules may or may not have a net charge of zero.

Examples of polar molecules with a net charge of zero include water and carbon monoxide (See Table 2.1). Note that uneven distribution of charge alone does not make for a dipole moment. Carbon dioxide has its electrons pulled closer to the oxygens, but since the oxygens are on exactly opposite sides of each other, the dipole moments cancel each other out.

The dipole moment () of a molecule is a measure of its polarity. This is given by = qx, where q is the charge and x is the distance between the fractional charges q- and q+ in the molecule.

Partial charges in a permanent dipole behave like fully ionic charges (opposites attract), but with less force. Permanent dipoles can be affected by ionic charges, by other permanent dipoles and by induced dipoles.

See also: Dispersion Forces, Induced Dipoles