Polarity of Molecules

1. Polarity of Molecules Objectives: • 1. State the two factors that determine polarity of a molecule. • 2. Explain how the structure of a molecule helps determine its properties. Key Terms: • Dipole, hydrogen bond, dispersion forces

2. Determining Polarity – bond polarity • Determining polarity requires looking at the type of bond as well as the shape of the molecule. • Bond polarity is determined by looking at the electronegativity differences between the atoms that are covalently bonded

3. Determining Polarity – single element • Molecules consisting of a single element are all non-polar. (O2, O3, H2, N2, Diamond, ...) • Chlorine (Cl2)

4. Determining Polarity – asymmetric electronegativities • If the electronegativities are dissimilar on the opposite sides of a molecule chances are that the molecule is polar • HCl, NH3, and CH3Cl are asymmetric • polar • BF3 and CCl4 are symmetric • Non-polar

5. Determining Polarity – axis of symmetry • Molecules that form mirror images (symmetrical) of themselves are non-polar even though their bonds appear polar. (CO2, CCl4, BF3, H3O+,…)

6. Determining Polarity – unshared electron pairs • Molecules that contain unpaired electrons are usually polar. Look at water and ammonia.

7. Intermolecular Forces • Vanderwahls Forces: • Dipole interactions • attraction of the positive and negative sides of a polar atom • Dispersion forces (temporary dipole) • temporary rearrangement of charge in non-polar atoms (weakest intermolecular force) • Hydrogen bonding • dipole interaction between a hydrogen that is already covalently bonded to O, N, or F and another negative diplole or ion (strongest intermolecular force) • important for determining the properties of water