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

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

Determining Polarity – single element

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

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

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+,…)

Determining Polarity – unshared electron pairs

• Molecules that contain unpaired electrons are usually polar. Look at water and ammonia.

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