Intermolecular forces

How are ionic compounds held together? Each ion is strongly attracted to the other ions in the crystal lattice because of alternating positive and negative charges

How are covalently bonded compounds (molecular compounds) held together?

Intermolecular forces Molecular compounds The molecules are held together by Intermolecular forces

What does “inter” mean? Interstate highway: a highway between 2 different states Interscholastic sports: sports competition between 2 different schools International treaty: an agreement between 2 different nations

What does intermolecular forces mean? Forces of attraction between 2 different molecules Molecule # 1 Molecule # 2

This is different than the BOND that hold the atoms in each individual molecule

If the force of attraction between two molecules is strong, the molecules will tend to “stick” together

If the force of attraction between two molecules is weak, the molecules will tend to “drift” apart

Weak Intermolecular Forces The weakest intermolecular force is called Van Der Waals Forces Non Polar molecules (diatomic elements) are held together by these weak forces

Weak Intermolecular Forces The weakest intermolecular force is called Van Der Waals Forces Most compounds held together by Van Der Waals forces are gases at room temperature: O2, N2 , H2 , F2 etc

Strong Intermolecular Forces A strong intermolecular force is called Dipole-dipole Force Polar molecules are held together by dipole –dipole forces

Strong Intermolecular Forces A strong intermolecular force is called Dipole-dipole Force + - + - The molecules “stick” together like magnets + - + - + -

+ - + Molecules held together by dipole-dipole forces have higher melting and boiling points - + - + - + -

HCl is a liquid at room temperature

The strongest intermolecular force …..unfortunately has a very misleading name: Hydrogen Bonding It is NOT an ionic, covalent, or metallic BOND

Hydrogen Bonding Is the attraction between a Hydrogen atom in one molecule, and an Oxygen, Nitrogen, or Fluorine atom in another molecule N O F H

Forming a hydrogen bond

Hydrogen bonding gives water its unusually high boiling point for such a small molecule