Geom03

Molecular Geometry and Bonding Theories

Polarity of Molecules

Polarity of Molecules

The "charge distribution" of a molecule is determined by

The shape of the molecule
The polarity of its bonds

A Polar Molecule:

The center of the overall negative charge on the molecule does not coincide with the center of overall positive charge on the molecule
The molecule can be oriented such that one end has a net negative charge and the other a net positive charge, i.e. the molecule is a dipole

A Nonpolar molecule

Has no charges on the opposite ends of the molecule
Or, has charges of the same sign on the opposite ends of the molecule
Molecule is not a dipole

Any diatomic molecule with a polar bond is a polar molecule (dipole)

Polar molecules align themselves:

in an electric field
with respect to one another
with respect to ions

The degree of polarity of a molecule is described by its dipole moment, m = Q * r

where

Q equals the charge on either end of the dipole

r is the distance between the charges

the greater the distance or the higher the charge, the greater the magnitude of the dipole

Dipole moments are generally reported in Debye units

1 debye = 3.33 x 10^-30 coulomb meters (C m)

Example: H-Cl a covalent polar compound

The H-Cl bond distance is 1.27Å
+1 and -1 charges in a dipole produce 1.60 x 10^-19 C

m = Qr = (1.60 x 10^-19 C)(1.27 x 10^-10 m)

m = 2.03 x 10^-29 C m

m = 2.03 x 10^-29 C m (1 debye/3.33 x 10^-30) = 6.10 debye

The actual dipole of H-Cl is 1.08 debye. The reason for this is that the compound is covalent and not ionic, thus the charges of the dipole are less that +1, and -1 (values expected for a fully ionic compound)

Compound	Bond Length (Å)	Electronegativity Difference	Dipole Moment (D)
HF	0.92	1.9	1.82
HCl	1.27	0.9	1.08
HBr	1.41	0.7	0.82
HI	1.61	0.4	0.44

Although the bond length is increasing, the dipole is decreasing as you move down the halogen group. The electronegativity decreases as we move down the group. Thus, the greater influence is the electronegativity of the two atoms (which influences the charge at the ends of the dipole).

The Polarity of Polyatomic Molecules

Each polar bond in a polyatomic molecule will have an associated dipole
The overall dipole of the molecule will be the sum of the individual dipoles

Although in carbon dioxide the oxygens have a partial negative charge and the carbon a partial positive charge, the molecule has no dipole - it will not orient in an electrical field
Water has a dipole and will orient in an electrical field

Although a polar bond is a prerequisite for a molecule to have a dipole, not all molecules with polar bonds exhibit dipoles

ABn molecules and non-polar geometries

For ABn molecules, where the central atom A is surrounded by identical atoms for B, there a certain molecular geometries which result in no effective dipole, regardless of how polar the individual bonds may be. These geometries are:

1996 Michael Blaber