1. Intermolecular forces

2. 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

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

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

5. 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

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

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

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

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

10. 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

12. 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

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

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

16. + - +
Molecules held together by dipole-dipole forces have higher melting and boiling points - + - + - + -

17. HCl is a liquid at room temperature

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

19. 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

20. Forming a hydrogen bond

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