Molecular Polarity Molecular Polarity Depends on 2 factors

Molecular Polarity

Molecular 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 & arrangement of bonds to determine overall polarity of molecule. CH 4 has 4 bonds.

Polarity 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 Molecules • Contain at least one mirror plane.

Symmetric vs. Asymmetric • CO 2 is nonpolar. • Can’t tell the ends apart. • H 2 O is bent. • Electron cloud is lopsided. • H 2 O is polar.

Polarity of CO 2? Draw an arrow along each bond pointing to the more electronegative atom. If the arrows cancel out, the molecule is NONPOLAR.

Polarity of H 2 O? The green arrows do not cancel out, so water is polar!

Symmetry of Larger Molecules CF 4 is fairly symmetric. Overall, it is nonpolar. The arrows cancel out.

Use the structural formula to predict Molecular Polarity! H H C H H

Ethane = C 2 H 6 Ethene = C 2 H 4 Ethyne = C 2 H 2 These molecules are symmetric and the electron cloud is the same on both 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