1. Molecular Geometry – Shapes!

2. Why does shape matter? Properties of molecules depend on:
Types of atoms present
Arrangement around the central atom
Polarity of Molecule is also determined by shape
Polarity will affect the interaction between molecules

3. VSEPR Theory “Valence Shell Electron Pair Repulsion”
# of shared pairs and lone pairs around CENTRAL atom used to determine overall shape
Theory states: “Repulsion between the sets of valence – level electrons surrounding an atom causes these sets to be as far apart as possible.”

4. How do we determine shapes??
VSEPR utilizes an ABE formula
A = central atom
B = atoms bonded to central atom
E = lone pairs of electrons on CENTRAL ATOM ONLY
Example:
H2O
AB2E2
Oxygen is central atom
2 hydrogen bonded to it (B2)
2 lone pairs on oxygen (E2)

5. What do I do with the formula
AB2E2
This shape is bent – from table on page 27A of your packet

6. Repulsion between two unshared pairs of electrons is greatest – push farthest apart
Repulsion between a shared and unshared pair of electrons is intermediate
Repulsion between two shared pairs of electrons is least

7. Follow the steps to get the ABE formula and shape
Back of first page of packet!!!
Draw the Lewis Dot structure for molecule
Check to see if central atom is an exception
Determine the number of atoms attached to central atom
# = subscript on B
Determine # of lone pairs on CENTRAL ATOM
# = subscript on E
Match the ABE formula you just determined to the shape on the chart (27A)

8. More Examples
CH4
NH3
HBr (first one on your sheet)

9. Polarity

10. What is Polarity???
Polarity arises when one of the atoms has the electrons more than the other atom
Unequal sharing
Red end represents area of greater electron density (electrons are there more often)

11. What is the result?? Bond has a positive and negative end
The end that is negative “sees” the electrons more than the positive side
Atom that has greater electronegativity will “see” electrons more

12. How do we determine if a bond is polar?
Difference in electronegativities
If the difference is > than 0.3, the bond is POLAR covalent
If the difference is < than 0.3, the bond is non polar covalent
Example: O-H bond
ΔEN = 3.5 – 2.1 = 1.4 POLAR!!!!
-δ +δ
O H
*oxygen is more electronegative so it ‘sees’ the shared electrons more
Points toward MORE EN atom

13. Other examples
Are these polar? If so, indicate the positive and negative end. O F H Cl B Si

14. Molecules can be polar too!
Molecules will be polar if:
Bonds are polar
AND
Molecule is NOT symmetric
*if a molecule has lone pair (nonbonding pairs) of electrons, automatically POLAR
USE FLOW CHART IN NOTES!!!!!

15. ΔEN = 1.0, polar bonds
CO2 – non polar, symmetrical
OVERALL , this molecule is not polar.
ΔEN = 1.4, polar bonds
H2O – POLAR, 2 lone pairs
OVERALL, this molecules IS polar

16. CCl4
ΔEN = 0.5, polar bonds
Symmetrical
NON POLAR!!!
CH3F
ΔEN = 1.5 and .4, polar bonds
Non Symmetrical
POLAR!!!

17. Both molecules are CH2Cl2. Which is Polar?
VS.
The molecule on the left is non symmetric (all negative pull is to one side) - POLAR