iiiiii i. lewis diagrams molecular structure. a. octet rule n remember… most atoms form bonds in...

I II III

I. Lewis Diagrams

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

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

B. Drawing Lewis Diagrams BeCl2

1 Be × 2e- = 2e-

2 Cl × 7e- = 14e-

16e-

Cl Be Cl - 4e-

12e-

B. Drawing Lewis Diagrams CO2

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-

NH4+

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

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

5 total

5 bond

0 lone

TRIGONAL BIPYRAMIDAL

120°/90°

PCl5

C. Common Molecular Shapes

6 total

6 bond

0 lone

OCTAHEDRAL

90°

SF6

C. Common Molecular Shapes

PF3

D. Examples

CO2

D. Examples

I II III

III. Molecular Polarity

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