Which Molecules Have Dipole Moments?

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Molecular Polarity Notes
Which Molecules Have Dipole Moments?

• Any two atom molecule (diatomic) with a polar
  bond.
• With three or more atoms (polyatomic) there are
  two considerations.
• There must be a polar bond. (H2O NH3)
• Geometry cant cancel it out.

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Dipole Moments

• A molecule with a center of negative charge and a
  center of positive charge is dipolar (two poles),
  or has a dipole moment.
• Center of charge doesnt have to be on an atom.
• Will line up in the presence of an electric
  field.

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When no electric field is present, the molecules
are randomly oriented. When the field is turned
on, the molecules tend to line up with their
negative ends toward the positive pole and their
positive ends toward the negative pole.
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C. Bond Polarity and Dipole Moments

• A dipole moment results when a polar molecule has
  a center for positive charge separate from a
  center for negative charge

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How It is drawn
The dipolar character of a molecule is often
represented by an arrow pointing to the negative
charge center with the tail of the arrow
indicating the positive center of charge
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A. Dipole Moment

• Direction of the polar bond in a molecule.
• Arrow points toward the more e-neg atom.

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B. Determining Molecular Polarity

• Depends on
• dipole moments
• molecular shape

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B. Determining Molecular Polarity

• Nonpolar Molecules
• Dipole moments are symmetrical and cancel out.

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B. Determining Molecular Polarity

• Polar Molecules
• Dipole moments are asymmetrical and dont cancel .

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B. Determining Molecular Polarity

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

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C. Bond Polarity and Dipole Moments

• Water molecule dipole moment

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C. Bond Polarity and Dipole Moments

• The polarity of water affects its properties

• Permits ionic compounds to dissolve in it

• Causes water to remain liquid at higher
  temperature

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Some Molecules Have Polar Bonds, but Do Not Have
Dipole Moment.

• This occurs when the individual bond polariteis
  are arranged in such a way that they cancel each
  other out.

EX CO2 molecule (oppose bond polarities cancel
out)
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Geometry and polarity

• Three shapes will cancel them out.
• Planar triangles

120º
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Geometry and polarity

• Three shapes will cancel them out.
• Tetrahedral

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Geometry and polarity

• Others dont cancel
• Bent

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Geometry and polarity

• Others dont cancel
• Trigonal Pyramidal