Intermolecular Forces Bonding Unit Intermolecular Forces

Intermolecular Forces Forces of attraction between different molecules rather than bonding forces within the same molecule.

Forces and Phases of Matter Substances with very little intermolecular attraction exist as gases Substances with strong intermolecular attraction exist as liquids Substances with very strong intermolecular (or ionic) attraction exist as solids

Polar Covalent Bond The two atoms do not share the electrons equally. One atom takes on a slight (partial) negative charge and the other atom taking on a partial positive charge. The molecule has a positive end and a negative end, called a dipole (think of a magnet). The little Greek symbol by the charges refers to a partial charge.

- water is a polar molecule because oxygen is more electronegative than hydrogen, and therefore electrons are pulled closer to oxygen.

These molecules are not polar.

How to tell if a molecule is polar.

Electronegativity Difference If the difference in electronegativities is between: 1.7 to 4.0: Ionic 0.3 to 1.7: Polar Covalent 0.0 to 0.3: Non-Polar Covalent Example: NaCl Na = 0.8, Cl = 3.0 Difference is 2.2, so this is an ionic bond!

Determining polarity of a molecule based on the symmetry of the molecule.

Types of Intermolecular Forces 1. Hydrogen Bond (strongest) Bonding between hydrogen and more electronegative neighboring atoms such as oxygen and nitrogen. IT IS NOT A BOND. A H … B or A & B are N, O, or F 11.2

Types of Intermolecular Forces 2. Ion-Dipole Forces Attractive forces between an ion and a polar molecule Ion-Dipole Interaction 11.2

11.2

Ion-Dipole Forces

Types of Intermolecular Forces 3. Dipole-Dipole Forces Attractive forces between polar molecules Orientation of Polar Molecules in a Solid 11.2

Types of Intermolecular Forces 4. Dispersion Forces – van der Walls forces/London forces (weakest) Attractive forces that arise as a result of temporary dipoles induced in atoms or molecules ion-induced dipole interaction dipole-induced dipole interaction 11.2

Summary: Intermolecular Forces Formed by the attraction between … Examples Ion-dipole an ion and a polar molecule Na+ and H2O Hydrogen bond molecules which have H on N, O, or F atoms H2O and H2O; H2O and CH3CH2OH Dipole - Dipole two polar molecules CH3Br and ICl; CH3Br and H2O Ion - Induced dipole an ion and a nonpolar Fe2+ and O2 Dipole - Induced dipole a polar molecule and a nonpolar molecule HCl and Cl2 London (dispersion) forces two nonpolar molecules CH4 and CH4; F2 and F2; CH4 and F2