How can you tell if a molecule is polar or not?

A molecule is polar if the bond dipoles do not cancel each other.

STEPS IN DECIDING POLARITY 1. Draw the Lewis structure. 2. Use theory to determine the 3D shape of the molecule. 3. Determine if the molecule is polar — in a polar molecule the bond dipoles do not cancel.

EXAMPLE

Is SF₅Cl a polar molecule?

Solution

The VSEPR shape of SF₅Cl is

The four equatorial S-F bond dipoles (wedges and dashed lines) cancel each other. The axial S-Cl and S-F bonds point in opposite directions and partially cancel each other. But F is more electronegative than Cl, so the S-F dipole is greater than the S-Cl dipole. There will be a net molecular dipole pointing “down”. The molecule is polar.

The following link gives a good review of polar molecules along with numerous practice problems.

http://www.tutor-homework.com/Chemistry_Help/Molecular_Geometry/Polar_Or_Nonpolar.html