1. Intermolecular Forces
Learning Goal 3: Part A

2. Intermolecular Forces
Forces of attraction between different molecules rather than bonding forces within the same molecule.
Dipole-dipole attraction
Hydrogen bonds
Dispersion forces

3. Forces and Phases of Matter
Substances with very little intermolecular attraction exist as gases
Substances with strong intermolecular attraction exist as liquids
Substances with very strong intermolecular (or ionic) attraction exist as solids

4. Types of Intermolecular Forces
1. Hydrogen Bond (strongest)
Bonding between hydrogen and more electronegative neighboring atoms such as oxygen and nitrogen.
IT IS NOT A BOND. A H … B or
A & B are N, O, or F
11.2

5. Polarity
A molecule, such as HF, that has a center of positive charge and a center of negative charge is said to be polar, or to have a dipole moment. H F + -

6. - water is a polar molecule because oxygen is more electronegative than hydrogen, and therefore electrons are pulled closer to oxygen.

9. Types of Intermolecular Forces
2. Ion-Dipole Forces
Attractive forces between an ion and a polar molecule
Ion-Dipole Interaction
11.2

10. Types of Intermolecular Forces
3. Dipole-Dipole Forces
Attractive forces between polar molecules
Orientation of Polar Molecules in a Solid
11.2

11. 11.2

12. Ion-Dipole Forces

13. Types of Intermolecular Forces
4. Dispersion Forces – van der Walls forces/London forces (weakest)
Attractive forces that arise as a result of temporary dipoles induced in atoms or molecules
ion-induced dipole interaction
dipole-induced dipole interaction
11.2

14. Summary: Intermolecular Forces
Formed by the
attraction between …
Examples
Ion-dipole
an ion and a polar molecule
Na+ and H2O
Hydrogen bond
molecules which have H on N, O, or F atoms
H2O and H2O; H2O
and CH3CH2OH
Dipole - Dipole
two polar molecules
CH3Br and ICl;
CH3Br and H2O
Ion - Induced dipole
an ion and a nonpolar
Fe2+ and O2
Dipole - Induced
dipole
a polar molecule and a
nonpolar molecule
HCl and Cl2
London (dispersion)
forces
two nonpolar
molecules
CH4 and CH4; F2 and F2; CH4 and F2

15. MOLECULAR GEOMETRY
Learning Goal #3 Part B

16. VSEPR MOLECULAR GEOMETRY Valence Shell Electron Pair Repulsion theory.
Molecule adopts the shape that minimizes the electron pair repulsions.
VSEPR
Valence Shell Electron Pair Repulsion theory.
Most important factor in determining geometry is relative repulsion between electron pairs.

17. Some Common Geometries
Linear
Trigonal Planar
Tetrahedral

18. VSEPR charts
Use the Lewis structure to determine the geometry of the molecule
Electron arrangement establishes the bond angles
Molecule takes the shape of that portion of the electron arrangement
Charts look at the CENTRAL atom for all data!

20. Structure Determination
by VSEPR
Water, H2O
The electron pair geometry is TETRAHEDRAL
2 bond pairs
2 lone pairs
The molecular geometry is BENT.

22. Methane, CH4

23. Ammonia, NH3

24. Water, H2O

25. Rules for Predicting Molecular Structure using the VSEPR Model
Two pairs of electron on a central atom in a molecule are always placed 180o apart (linear).
Three pairs of electrons on a central atom in a molecule are always placed 120o apart on the same plane as the central atom (trigonal Planar).
Four pairs of electrons on a central atom in a molecule are always placed 109.5o apart (tetrahedral).

26. Rules for Predicting Molecular Structure using the VSEPR Model
When every pair of electrons on the central atom is shared with another atom, the molecular structure has the same name as the arrangement of the electron pairs.
Number of Pairs
Name of Arrangement 2
Linear 3
Trigonal Planar 4
tetrahedral

27. Rules for Predicting Molecular Structure using the VSEPR Model
When one or more of the electron pairs around a central atom are lone pairs, the name for the molecular structure is different from the arrangement of electron pairs.

28. Determining polarity of a molecule based on the symmetry of the molecule.