1. Aim: What attracts molecules to each other?
Do Now: Take out the homework. In Pairs discuss and correct your answers.

2. Review Molecular Polarity

3. Questions
What holds the hydrogen and the chlorine together to form HCl?
What holds the multiple HCl molecules together?

4. Discuss in Pairs
How would you define an intramolecular attraction?
How would you define an intermolecular attraction?
Which attraction is stronger?

5. Intramolecular forces vs Intermolecular forces
Intramolecular forces are the forces that hold atoms together within a molecule (bonds). Intermolecular forces are forces that exist between molecules.

6. Intermolecular Forces (IMF)
Intermolecular forces are the forces of attraction between molecules
The strength of these forces at a given temperature determine whether a substance will have the properties of a solid, a liquid, or a gas.
Question: At room temperature, which substance has stronger attractions between its molecules? A solid or a liquid? Consider the relative distance between the particles in the solid and liquid phase.

7. Intramolecular forces vs Intermolecular forces
Ionic bonds
Metallic bonds
Covalent bonds
Polar covalent bond
Nonpolar covalent bond
Dipole-dipole
Hydrogen bonding
Dispersion
Other types of attractions:
Ion-dipole
network

8. Ion- Ion Attraction (Ionic Bonding)
Ion-ion interactions occur between ions of opposite charges, resulting in solids with ionic bonds.

9. Dipole-Dipole Interaction
Class discussion: Predict how two dipole molecules are attracted to each other.

10. Dipole-Dipole interactions
Occur when polar molecules that are attracted to each other.
Opposite ends of dipoles attract, but not completely hooked like in ionic solids (ion-ion attraction).
H F
d+ d-
H F
d+ d-
||||||||||

11. Dipole-Dipole Interactions
d+ d-
d+ d-
d+ d-
d+ d-
d+ d-
d+ d-
d+ d-
d+ d-

12. Hydrogen Bond Special kind of dipole-dipole interaction
Occurs specifically between a hydrogen atom bonded to either an oxygen, nitrogen, or fluorine atom.

13. Hydrogen bonding
Are the attractive force caused by hydrogen bonded to F, O, or N.
F, O, and N are very electronegative so it is a very strong dipole. (very electronegative and have small atomic radii)
The strongest of the intermolecular forces.

14. Hydrogen bonding between water molecules
Water H2O has high melting and boiling points because of the hydrogen bonds between the water molecules. H O H O H O H O H O

15. Hydrogen Bonding between ammonia (NH3) molecules

16. Ion-Dipole Attraction
Class discussion: Predict how a lithium ion would be attracted to a dipole molecule.

17. Ion-Dipole Attraction
The attraction between the ion of an ionic compound, such as salt, and molecules of water (or other polar liquids).
When you put salt in water, the positive ion Na+ is attracted to the negative end of the water molecule, and the negative ion Cl- is attracted to the positive end of water.
ectfolder/flashfiles/thermochem/solutionSalt.html

18. Ion-Dipole Interaction

19. Dispersion Forces
Occur between nonpolar molecules. Electrons happen to momentarily move to one end of the molecule resulting in a temporary dipole.
Depend on the number of electrons
Bigger molecules have more electrons resulting in stronger forces, and decreasing the distance between the molecules. Explains why:
Fluorine F2 is a gas
Bromine Br2 is a liquid
Iodine I2 is a solid

20. Dispersion Forces
Dispersion forces even occur between monatomic molecules - molecules of noble gases, like helium, which consist of a single atom

21. Network Solids
A Network Solid contains atoms that are all covalently bonded to each other.
Carbon can form network solids. Ex: diamond and graphite

22. Relative strength of intermolecular forces of attraction
Occurs between …
Relative strength
Dipole-dipole attraction
Partially oppositely charged ions
Strongest
Hydrogen bonding
H atom and N, O, or F atom
As strong as dipole-dipole attraction
Dispersion attraction
Temporary or induced dipoles
Weakest

23. How do Intermolecular Forces affect Physical Properties?
They are what make solid and liquid molecular compounds possible.
The stronger the attractions, the closer the molecules become.
They contribute to the temperature at which a substance boils/melts
The stronger the attraction, the higher the b.p. / m.p.

24. How forces of attraction affect properties of compounds
Type of compound
Intermolecular forces present
Relative order of boiling and melting points
Ionic compounds
Ion to ion attraction between ions
1, highest)
Covalent compounds containing hydrogen bonds
Hydrogen bonds 2
Polar covalent compounds
Dipole-dipole attraction between dipoles created by partially charged atoms 3
Nonpolar covalent compounds
Dispersion forces
4, lowest

26. Practice Question 1
Which type of bonding accounts for the unusually high boiling points of water?
Ionic bonding
Covalent bonding
Hydrogen bonding
Network bonding
The attraction between water molecules is called hydrogen bonding. Hydrogen bonds are relatively strong. The stronger the intermolecular force the higher the melting a boiling boints.

27. Practice Question 2
The attractions that allow molecules of krypton to exist in the solid phase are due to
Ionic bonds
Covalent bonds
Molecule- ion forces
Van der Waals forces (dispersion forces)
Krypton does not have any poles (does not have positive or negative end). However, temporarily the electrons in krypton will shift to one side, resulting in a temporary dipole. The attraction between two temporary dipoles is a Vand der Waals force.

28. Practice Question 3
The dispersion forces of attraction between molecules always become stronger as molecular size
increases, and the distance between the molecules increases
Increases, and the distance between the molecules decreases
Decreases, and the distance between the molecules increases
Decreases, and the distance between the molecules decreases
Bigger molecules and decrease in distance between molecules More electrons stronger forces