1. Section 6.5 – Molecular Geometry
The properties of molecules depend on the bonding and the molecular geometry, the 3- dimensional arrangement of the atoms in space.

2. Molecular Polarity
This is the uneven distribution of molecular charge, and it is determined by the polarity of each bond, along with the geometry of the molecule.

3. Molecular Polarity and Symmetry
2 or more lines of symmetry  Nonpolar molecule; electrons distributed symmetrically
Only 1 line of symmetry  polar molecule; asymmetrical electron distribution creates a dipole
H H
H Cl
Cl Cl
Non-polar
Polar
Non-polar

4. Dipole
A dipole is created by equal but opposite charges that are separated by a short distance
We show a dipole using an arrow pointing towards the more electronegative atom
δ+ indicates the partial positive charge, and δ- indicates the partial negative charge
H H δ+
H Cl δ-
Cl Cl
Non-polar
Polar
Non-polar

5. Identify the polar molecules, and indicate the dipole…δ+ H H Cl
H C H
Cl C Cl δ-
Polar
Non-polar

6. Identify the polar molecules, and indicate the dipole….. δ- ..
H O H δ-
H N H .. H δ+
Polar δ+
Polar

7. Dipoles
Give each molecule oppositely charged ends that can be attracted to each other.
As the electronegativity difference increases, attractive force strength increases = stronger dipoles.
H Cl
Polar δ+ δ-
2.1
3.0 δ+ δ-
H F
2.1
4.0
polar

8. Intermolecular Forces
These are the forces of attraction that occur between molecules. They vary in strength but are generally weaker than regular bonding (ionic, covalent, or metallic).

9. Intermolecular Forces
Are responsible for holding molecules together in the solid and liquid phase
Caused by the attraction of the + end of one molecule to the – end of another molecule

10. What do you notice? (p.190) Nonpolar covalent H2 -253 O2 -183 Cl2 -34
Bonding Type
Substance
BP (Celcius)
Nonpolar covalent H2
-253 O2
-183
Cl2
-34
C6H6 80
Polar Covalent
PH3
-88
NH3
-33
H2O
100 HF 20
Ionic
NaCl
1413
MgF2
2239
Metallic Cu
2567 Fe
2750 W
5600

11. Melting and Boiling Points
a measure of the force of attraction
between molecules in covalent compounds
between ions in ionic compounds
and between atoms in metals

12. Melting and Boiling Points
As a substance is heated, the kinetic energy of its particles increases.
At the MP, the energy is enough to overcome the attractive forces holding the particles in fixed positions (in a solid). The particles are able to move/flow past each other, but the particles are still held close together.

13. Melting and Boiling Points
As a substance is heated, the kinetic energy of its particles increases.
At the BP, the energy is enough to overcome the force of attraction holding the particles close together in the liquid. The particles are able to pull away from each other and enter the gas phase.

14. Melting and Boiling Points
The higher the boiling point, the stronger the forces between particles.
Remember particles means molecules for covalent substances, ions for ionic substances, atoms for metals

15. 3 Types of Intermolecular Forces
Dipole – Dipole Forces
Hydrogen bonding (special case of D-D)
London Dispersion Forces

16. Dipole Force
The strongest intermolecular forces exist between polar molecules.
The δ+ end of one molecule is attracted to the δ- end of another molecule.
The direction of the arrow
is pointed to the negative pole,
the crossed tail indicates the + side

17. Dipole-dipole forces
These are the forces of attraction between polar molecules.
Example:
bp for F2 is -188°C
bp for HF is 20°C
bp for HCl is -85°C
Which is the stronger
dipole-dipole force?

18. Ion-dipole forces
These are the forces of attraction between an ion and the partial charge on a polar molecule.
This explains why ionic compounds dissolve in polar liquids.
Generally stronger than dipole-dipole forces

19. Induced Dipoles
The electrons of a nonpolar molecule can be temporarily attracted by a polar molecule.
This is weaker than a regular dipole-dipole force.
This can be very important in the
solubility of gases in water.

20. Usually Represented By
Hydrogen Bonding
A very special type of dipole-dipole force in which a hydrogen atom that is bonded to a highly electronegative atom is attracted to an unshared pair of electrons of another molecule.
Usually Represented By
Dotted Lines

21. Usually Represented By
Hydrogen Bonding
Only happens in molecules where H is bonded to O, F, or N, and sometimes Cl
Usually Represented By
Dotted Lines

22. Explains High bp’s of Some Compounds
Gives the H atom a large positive charge, and it’s small size allows it to come very close to the unshared pair of electrons on an adjacent molecule.

23. Explains properties of water
High surface tension
Why ice floats on water

24. Extremely Important in Biochemistry

25. London Dispersion Forces
This is a weak attractive force resulting from electron motion and the creation of an instantaneous dipole.
Important for noble gases and nonpolar molecules.
Explains why nonpolar gases can be liquefied.

26. London Dispersion forces
Act between all atoms and molecules
However, dispersion forces are much weaker than dipole-dipole forces
Dispersion forces are the only forces acting among noble-gas atoms, and non-polar molecules.

27. increase with increasing atomic or molar mass.
Dispersion forces
Substance
Bp (1atm, °C) H2
-253 O2
-183
Cl2
-34 He
-268 Ar
-185
Increase with the increased number of electrons in the interacting atoms or molecules
increase with increasing atomic or molar mass.

28. Recall the bonding lab and evaporation rates…
Explain why pentane and hexane evaporated quickly, but the other substances did not.
Explain why pentane evaporated more quickly than hexane.
Hexane, C6H14 (covalent) ~ 40 sec
Water, H2O (covalent) > 5 min.
Acetic Acid, (C2H4O2) (covalent) > 5 min.
Pentane, C5H12 (covalent) ~ 25 sec
Isopropyl Alcohol, C3H8O (covalent) > 5 min.