Molecular Polarity AP Chemistry

Polar vs. Non-Polar Molecules 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.

When determining the polarity of a molecule, follow these steps: Draw the Electron Dot structure of the molecule. Using the electronegativity chart determine the difference in electronegativity for each bond. 0—0.4 = Non-polar 0.5—1.7 = Polar

The molecule is non-polar if : 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)

The molecule is polar if: There is a net dipole moment each 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 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 Debye Bond dipole moment is a vector quantity – it has a magnitude and direction. Molecular dipole moment = vector sum of all bond moments

+ - Dipole Moment H Cl Direction of the polar bond in a molecule. Arrow points toward the more e-neg atom. H Cl + -

Nonpolar Molecules BF3 Dipole moments are symmetrical and cancel out.

Polar Molecules H2O Dipole moments are asymmetrical and don’t cancel . Molecule has a net dipole moment. H2O H O net dipole moment

Determining Molecular Polarity Depends on: dipole moments molecular shape

Polar Bonds vs. Polar Molecules The effect of polar bonds on the polarity of the entire molecule depends on the molecule shape carbon dioxide has two polar bonds, and is linear = nonpolar molecule!

Polar molecules The effect of polar bonds on the polarity of the entire molecule depends on the molecule shape water has two polar bonds and a bent shape; the highly electronegative oxygen pulls the e- away from H = very polar!

Determining Molecular Polarity Therefore, polar molecules have... asymmetrical shape (lone pairs) or asymmetrical atoms CHCl3 H Cl net dipole moment