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Molecular Polarity

Molecular Polarity . 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.

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Molecular Polarity

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  1. Molecular Polarity

  2. 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.

  3. 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

  4. 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)

  5. 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.

  6. 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

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

  8. F BF3 B F F Nonpolar Molecules • Dipole moments are symmetrical and cancel out.

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

  10. Determining Molecular Polarity • Depends on: • dipole moments • molecular shape

  11. 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!

  12. 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!

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

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