Molecular Geometry – Shapes!

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

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.”

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)

What do I do with the formula

• AB2E2

• This shape is bent – from table on page 27A of your packet

• 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

Follow the steps to get the ABE formula and shape

• Back of first page of packet!!!A. Draw the Lewis Dot structure for moleculeB. Check to see if central atom is an exceptionC. Determine the number of atoms attached to central

atomA. # = subscript on BD. Determine # of lone pairs on CENTRAL ATOMA. # = subscript on EE. Match the ABE formula you just determined to the

shape on the chart (27A)

More Examples

• CH4

• NH3

• HBr (first one on your sheet)

Polarity

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)

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

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

Other examples

Are these polar? If so, indicate the positive and negative end.

O F

H Cl

B Si

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!!!!!

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

• CCl4

• ΔEN = 0.5, polar bonds• Symmetrical• NON POLAR!!!

• CH3F

• ΔEN = 1.5 and .4, polar bonds

• Non Symmetrical• POLAR!!!

Both molecules are CH2Cl2. Which is Polar?

• The molecule on the left is non symmetric (all negative pull is to one side) - POLAR

VS.