Molecular Polarity

MolecularMolecular Polarity Polarity

• Depends on 2 factors.–Type of bonds in the molecule

–Arrangement of bonds or shape of molecule

Diatomic Molecules

• Simplest case:

• Bond Polarity and Molecular Polarity are identical

• Larger Molecules – Have to look at how the bonds are arranged.

For larger molecules, look at the kind For larger molecules, look at the kind & arrangement of bonds to determine & arrangement of bonds to determine overall polarity of molecule.overall polarity of molecule.

CHCH44 has 4 bonds. has 4 bonds.

Polarity of MoleculesPolarity of Molecules

• A molecule may contain polar bonds, but not be polar! Depends on the geometry of the molecule.

• If molecule is symmetric, the “pull” of one polar bond is offset by the “pull” of another polar bond.– It’s a tug-of-war that no one can win!

Symmetric MoleculesSymmetric Molecules

• Contain at least one mirror plane.

Symmetric vs. Asymmetric

• CO2 is nonpolar.

• Can’t tell the ends apart.

• H2O is bent.• Electron cloud is lopsided.• H2O is polar.

Polarity of CO2?

Draw an arrow along each bond pointing Draw an arrow along each bond pointing to the more electronegative atom.to the more electronegative atom.If the arrows cancel out, the molecule is If the arrows cancel out, the molecule is NONPOLAR.NONPOLAR.

Polarity of H2O?

The green The green arrows do not arrows do not cancel out, so cancel out, so water is polar!water is polar!

Symmetry of Larger MoleculesSymmetry of Larger Molecules

CFCF44 is fairly is fairly

symmetric. symmetric. Overall, it is Overall, it is nonpolar.nonpolar.

The arrows The arrows cancel out.cancel out.

Use the structural formula to predict Molecular Polarity!

HH

H H C C H H HH

Ethane = CEthane = C22HH66

Ethene = CEthene = C22HH44

Ethyne = CEthyne = C22HH22

These molecules are symmetric and These molecules are symmetric and the electron cloud is the same on both the electron cloud is the same on both ends. Overall, they are nonpolar.ends. Overall, they are nonpolar.

Molecular Polarity

• If you know the shape, you can use the arrow technique to determine the polarity.

• So how do you get the shape?

Polarity

– Linear molecules• Nonpolar if the ends are the same• Polar if the ends are different

– Tetrahedral• Nonpolar if all 4 corners are the same• Polar if even 1 corner is different

– Trigonal Pyramids• Always polar

– Bent molecules• Always polar