1. Intermolecular Forces

2. What are intermolecular forces? NOT chemical bonds, less strength Attractive forces between molecules Molecular level, not individual atoms Types Dipole-Dipole London Dispersion Hydrogen Bonding

3. 1. Dipole-Dipole Forces Found in polar molecules Dipole—equal and opposite charge separation over a short distance in a molecule Charge separation occurring within a molecule, force of attraction between molecules Acts on adjacent molecules, opposite charges on “neighboring” molecules attract

6. Ex. HCl

7. 2. London Dispersion Forces TEMPORARILY uneven charge distribution Found between ALL atoms/molecules at varying degrees, some occur more frequently than others At SOME point in time, electron density is greater around one atom than the other Induces temporary dipoles in adjacent molecules

8. London Dispersion Forces (cont.) Mobile electron density Molecular shape influences the strength of dispersion forces within a molecule Melting and boiling points indicative of intermolecular force strength

9. Penny Lab !

10. 3. Hydrogen Bonding Strong dipole-dipole force Hydrogen attached to electronegative atom is attracted to UNSHARED electron pairs of another nearby electronegative atom (book definition) Dotted lines represent hydrogen bonding

11. Hydrogen Bonding (cont.) Considered a FORCE, not a type of bond Connects 2 electronegative atoms together Force holding water molecules together Results from a negative charge on an atom and a positive charge on hydrogen H-X format Ex. Hydrogen bonding between DNA base pairs of double helix

12. Example: H 2 O

13. Unique Properties of Water Freezes and expands as solid Solid is less dense than liquid (ex. Ice floats) High melting point High boiling point Highest surface tension (Hg only exception) Other compounds can easily dissolve in it High specific heat

14. ALL DUE TO HYDROGEN BONDING !