Molecular PolarityMolecular Polarity

AP ChemistryAP Chemistry

Polar vs. Non-Polar MoleculesPolar vs. Non-Polar MoleculesPolarity in a molecules determines Polarity in a molecules determines whether or not electrons in that whether or not electrons in that molecule are shared equally.molecule are shared equally.

When determining the polarity of a When determining the polarity of a molecule, it is all about molecule, it is all about symmetry.symmetry.

Asymmetric molecules tend to be polar. Asymmetric molecules tend to be polar.

Symmetric molecules are always non-Symmetric molecules are always non-polar. polar.

When determining the polarity of a When determining the polarity of a molecule, follow these steps:molecule, follow these steps:

Draw the Electron Dot structure of the Draw the Electron Dot structure of the molecule.molecule.

Using the electronegativity chart determine Using the electronegativity chart determine the difference in electronegativity for each the difference in electronegativity for each bond.bond.

0—0.4 = Non-polar0—0.4 = Non-polar

0.5—1.7 = Polar0.5—1.7 = Polar

The molecule is The molecule is non-polarnon-polar if : if :

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

The molecule is The molecule is polarpolar if: if:

There is a net dipole momentThere is a net dipole moment

each bond in the molecule is non-polar, each bond in the molecule is non-polar, but there but there areare unbonded electron pairs on unbonded electron pairs on the central atom.the central atom.

bonds in the molecule have different bonds in the molecule have different polarities and/or there polarities and/or there

areare unbonded electron pairs on the unbonded electron pairs on the central atom.central atom.

Dipole MomentsDipole Moments

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

Dipole MomentDipole Moment

Direction of the polar bond in a molecule.Direction of the polar bond in a molecule.

Arrow points toward the more eArrow points toward the more e--neg atom.neg atom.

H Cl+ -

Nonpolar MoleculesNonpolar Molecules

Dipole moments are symmetrical and Dipole moments are symmetrical and cancel out.cancel out.

BF3

F

F F

B

Polar MoleculesPolar Molecules

– Dipole moments are asymmetrical and Dipole moments are asymmetrical and don’t cancel .don’t cancel .

– Molecule has a net dipole moment.Molecule has a net dipole moment.

netdipolemoment

H2OH H

O

Determining Molecular PolarityDetermining Molecular Polarity

Depends onDepends on::–dipole momentsdipole moments–molecular shapemolecular shape

Polar Bonds vs. Polar MoleculesPolar Bonds vs. Polar MoleculesThe effect of polar bonds on the The effect of polar bonds on the polarity of the entire molecule polarity of the entire molecule depends on the depends on the molecule shapemolecule shape– carbon dioxide has two polar bonds, and carbon dioxide has two polar bonds, and

is linear = is linear = nonpolar moleculenonpolar molecule!!

Polar moleculesPolar moleculesThe effect of polar bonds on the The effect of polar bonds on the polarity of the entire molecule depends polarity of the entire molecule depends on the on the molecule shapemolecule shape– water has two polar bonds and a bent shape; water has two polar bonds and a bent shape;

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

CHCl3

H

Cl ClCl

Determining Molecular PolarityDetermining Molecular Polarity

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

netdipolemoment