Polar Molecules

H F

electron richregion

electron poorregion

Dipole moment (µ)?

- A measure of molecular polarity

- A polar molecule has µ ≠ 0 Non-polar molecule has µ = 0

= q x r

q = charge (coulomb)

r = the distance between charges (m)

•The direction of polarity of a polar bond can be symbolised by a vector quantity

( )

•The crossed end of the arrow is the positive end and the arrow head is the negative end.

H Cl

Example:

Covalent bond between same atoms

Covalent bond between atoms of the same element is nonpolar

→ nonpolar molecule.

Bonding electrons are shared equally. Example:

Covalent bond between different atoms

Covalent bond between different atoms is polar. - bonding electrons closer to the more

electronegative atom.

However, molecules could be polar or nonpolar, depending on its shape.

Determining polar and nonpolar molecules

Use the three-step approach outlined below : Use electronegativity values to predict bond

dipoles. Use the VSEPR method to predict the

molecular shape. From the molecular shape, determine

whether bond dipoles cancel to give a non-polar molecule or combine to produce a (resultant dipole moment) for the molecule.

Example:

Molecules with different atoms & asymmetrically arranged are polar.

Molecules with different atoms, symmetrically arranged but having µ = 0 are nonpolar.

In CCl4, each bond is polar.

The molecule as a whole, however, is nonpolar

Example:

Example:

In SF6, each bond is polar.

The molecule is nonpolar

The molecule is polar because µ ≠ 0 .The dipole moment arises because the differencein electronegativity.

Example:

Molecules with different atoms, symmetrically arranged but having µ ≠ 0 are polar.

Chloromethane ( CH3Cl)

H

H

Cl

C

H

The molecule is polar because µ ≠ 0 .The dipole moment arises because the differencein electronegativity.

Example:

Molecules with lone pairs on the central atom

Note: Molecules which have lone pairs are usually polar.

Let us consider the molecules in which there are lone pairs on the central atoms.

Ammonia( effect of lone pairs )

0

Example:

O

Which of the following molecules have a dipole moment?H2O, CO2, SO2, and CH4

HH

dipole momentpolar molecule

SO

O

CO O

no dipole momentnonpolar molecule

dipole momentpolar molecule

C

H

H

HH

no dipole momentnonpolar molecule

Indicate the direction in which the shared electron pair is shifted in the molecules givenbelow.

Exercise 1

Answer:

Exercise 2

Predicting the polarity of molecules.

State which of them are polar and which are nonpolar.

a) H2O

b) NH3

c) CCl4

d) CHCl3

Answer: