Chemistry

Brodersen 2013/2014

Polarity in a molecules determines whether or not electrons in that molecule are shared equally.

When determining the polarity of a molecule, it is all about symmetry. Asymmetric molecules tend to be polar. Symmetric molecules are always non-polar.

Draw the Electron Dot structure of the molecule.Using the electronegativity chart determine the difference in electronegativity for each bond.0—0.4 = Non-polar0.5—1.9 = Polar2.0 and greater = Ionic

each bond in the molecule is non-polar and there are no unbonded electron pairs.each bond in the molecule has the same polarity and there are no unbonded electron pairs on the central atom.There is no net dipole moment (all moments cancel out)

There is a net dipole momenteach bond in the molecule is non-polar, but there are unbonded electron pairs on the central atom.bonds in the molecule have different polarities and/or there are unbonded electron pairs on the central atom.

Dipole Moments (μ) - a quantitative measure of the polarity of a bond.μ = charge x distance = Q x r (distance is in meters)Dipole moment units are the DebyeBond dipole moment is a vector quantity – it has a magnitude and direction.Molecular dipole moment = vector sum of all bond moments

Direction of the polar bond in a molecule.

Arrow points toward the more e-neg atom.

H Cl+ -

Dipole moments are symmetrical and cancel out.

BF3

F

F F

B

Dipole moments are asymmetrical and don’t cancel .

Molecule has a net dipole moment.

netdipolemoment

H2OH H

O

Depends on:dipole momentsmolecular shape

The effect of polar bonds on the polarity of the entire molecule depends on the molecule shapecarbon dioxide has two polar bonds, and is linear = nonpolar molecule!

The effect of polar bonds on the polarity of the entire molecule depends on the molecule shapewater has two polar bonds and a bent shape; the

highly electronegative oxygen pulls the e- away from H = very polar!

CHCl3

H

Cl ClCl

Therefore, polar molecules have...asymmetrical shape (lone pairs) or asymmetrical atoms

netdipolemoment