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Which Molecules Have Dipole Moments? Any two atom molecule (diatomic) with a polar bond. With three or more atoms (polyatomic) there are two considerations.

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Presentation on theme: "Which Molecules Have Dipole Moments? Any two atom molecule (diatomic) with a polar bond. With three or more atoms (polyatomic) there are two considerations."— Presentation transcript:

1 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. (H 2 O & NH 3 ) Geometry cant cancel it out. Molecular Polarity Notes

2 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.

3 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.

4 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

5 How It is drawn H - F + - 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

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

7 B. Determining Molecular Polarity Depends on: –dipole moments –molecular shape

8 B. Determining Molecular Polarity Nonpolar Molecules –Dipole moments are symmetrical and cancel out. BF 3 F F F B

9 B. Determining Molecular Polarity Polar Molecules –Dipole moments are asymmetrical and dont cancel. net dipole moment H2OH2O H H O

10 CHCl 3 H Cl B. Determining Molecular Polarity Therefore, polar molecules have... –asymmetrical shape (lone pairs) or –asymmetrical atoms net dipole moment

11 C. Bond Polarity and Dipole Moments Water molecule dipole moment

12 C. Bond Polarity and Dipole Moments The polarity of water affects its properties –Causes water to remain liquid at higher temperature –Permits ionic compounds to dissolve in it

13 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: CO 2 molecule (oppose bond polarities cancel out)

14 Geometry and polarity Three shapes will cancel them out. Planar triangles 120º

15 Geometry and polarity Three shapes will cancel them out. Tetrahedral

16 Geometry and polarity Others dont cancel Bent

17 Geometry and polarity Others dont cancel Trigonal Pyramidal


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