I II III

I. Lewis Diagrams

(p. 184-189)

Ch. 6 – Molecular Structure

A. Octet Rule

Remember…· Most atoms form bonds in order to

have 8 valence electrons.

· Hydrogen 2 valence e-

· Groups 1,2,3 get 2,4,6 valence e-

· Expanded octet more than 8

valence e- (e.g. S, P, Xe)

· Radicals odd # of valence e-

Exceptions:

A. Octet Rule

F B FFH O HN O

Very unstable!!

F FF S FF F

B. Drawing Lewis Diagrams Find total # of valence e-. Arrange atoms - singular atom is

usually in the middle. Form bonds between atoms (2 e-). Distribute remaining e- to give each

atom an octet (recall exceptions). If there aren’t enough e- to go around,

form double or triple bonds.

B. Drawing Lewis Diagrams CF4

1 C × 4e- = 4e-

4 F × 7e- = 28e-

32e- FF C F

F

- 8e-

24e-

B. Drawing Lewis Diagrams BeCl2

1 Be × 2e- = 2e-

2 Cl × 7e- = 14e-

16e-

Cl Be Cl - 4e-

12e-

B. Drawing Lewis Diagrams CO2

1 C × 4e- = 4e-

2 O × 6e- = 12e-

16e-

O C O - 4e-

12e-

C. Polyatomic Ions

To find total # of valence e-:· Add 1e- for each negative charge.· Subtract 1e- for each positive

charge. Place brackets around the ion and

label the charge.

C. Polyatomic Ions

ClO4-

1 Cl × 7e- = 7e-

4 O × 6e- = 24e-

31e- OO Cl O

O

+ 1e-

32e-

- 8e-

24e-

NH4+

1 N × 5e- = 5e-

4 H × 1e- = 4e-

9e- HH N H

H

- 1e-

8e-

- 8e-

0e-

C. Polyatomic Ions

D. Resonance Structures

Molecules that can’t be correctly represented by a single Lewis diagram.

Actual structure is an average of all the possibilities.

Show possible structures separated by a double-headed arrow.

D. Resonance Structures

OO S O

OO S O

OO S O

SO3

I II III

II. Molecular Geometry

(p. 197-200)

Ch. 6 – Molecular Structure

A. VSEPR Theory

Valence Shell Electron Pair Repulsion Theory

Electron pairs orient themselves in order to minimize repulsive forces.

A. VSEPR Theory

Types of e- Pairs· Bonding pairs - form bonds· Lone pairs - nonbonding e-

Lone pairs repel

more strongly than

bonding pairs!!!

A. VSEPR Theory Lone pairs reduce the bond angle

between atoms.

Bond Angle

Draw the Lewis Diagram. Tally up e- pairs on central atom.

· double/triple bonds = ONE pair Shape is determined by the # of

bonding pairs and lone pairs.

Know the 8 common shapes & their bond angles!

B. Determining Molecular Shape

C. Common Molecular Shapes

2 total

2 bond

0 lone

LINEAR180°BeH2

3 total

3 bond

0 lone

TRIGONAL PLANAR

120°

BF3

C. Common Molecular Shapes

C. Common Molecular Shapes

3 total

2 bond

1 lone

BENT

<120°

SO2

4 total

4 bond

0 lone

TETRAHEDRAL

109.5°

CH4

C. Common Molecular Shapes

4 total

3 bond

1 lone

TRIGONAL PYRAMIDAL

107°

NH3

C. Common Molecular Shapes

4 total

2 bond

2 lone

BENT

104.5°

H2O

C. Common Molecular Shapes

PF3

4 total

3 bond

1 lone

TRIGONAL PYRAMIDAL

107°

F P FF

D. Examples

CO2

O C O2 total

2 bond

0 loneLINEAR

180°

D. Examples

I II III

III. Polarity & IMF

(p. 204-207)

Ch. 6 – Molecular Structure

A. Dipole Moment

Direction of the polar bond in a molecule.

Arrow points toward the more e-neg atom.

H Cl+ -

B. Determining Molecular Polarity Depends on:

· dipole moments· molecular shape

B. Determining Molecular Polarity Nonpolar Molecules

· Dipole moments are symmetrical and cancel out.

BF3

F

F F

B

B. Determining Molecular Polarity Polar Molecules

· Dipole moments are asymmetrical and don’t cancel .

netdipolemoment

H2OH H

O

CHCl3

H

Cl ClCl

B. Determining Molecular Polarity Therefore, polar molecules have...

· asymmetrical shape (lone pairs) or · asymmetrical atoms

netdipolemoment

Dipole-Dipole Forces

Attractive forces between polar covalent molecules

London (Dispersion) Forces Attractive forces between the electron

clouds of large molecules in large quantity

Larger mass = Larger London Forces

Hydrogen Bonding

Special dipole-dipole attraction that involves H bonded with high electronegative elements N, O, or F