# IIIIII I. Lewis Diagrams Molecular Structure. A. Octet Rule n Remember…  Most atoms form bonds in order to have 8 valence electrons.

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IIIIII I. Lewis Diagrams Molecular Structure

A. Octet Rule n 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 - n Exceptions: A. Octet Rule

B. Drawing Lewis Diagrams n Find total # of valence e -. n Arrange atoms - singular atom is usually in the middle. n Form bonds between atoms (2 e - ). n Distribute remaining e - to give each atom an octet (recall exceptions). n If there aren’t enough e - to go around, form double or triple bonds.

B. Drawing Lewis Diagrams n CF 4

B. Drawing Lewis Diagrams n BeCl 2 1 Be × 2e - = 2e - 2 Cl × 7e - = 14e - 16e - Cl Be Cl - 4e - 12e -

B. Drawing Lewis Diagrams n CO 2

C. Polyatomic Ions n To find total # of valence e - :  Add 1e - for each negative charge.  Subtract 1e - for each positive charge. n Place brackets around the ion and label the charge.

C. Polyatomic Ions n ClO 4 -

n NH 4 + C. Polyatomic Ions

D. Resonance Structures n Molecules that can’t be correctly represented by a single Lewis diagram. n Actual structure is an average of all the possibilities. n Show possible structures separated by a double-headed arrow.

D. Resonance Structures O O S O O O S O O O S O n SO 3

IIIIII II. Molecular Geometry Molecular Structure

A. VSEPR Theory n Valence Shell Electron Pair Repulsion Theory n Electron pairs orient themselves in order to minimize repulsive forces.

A. VSEPR Theory n Types of e - Pairs  Bonding pairs - form bonds  Lone pairs - nonbonding e - Lone pairs repel more strongly than bonding pairs!!!

A. VSEPR Theory n Lone pairs reduce the bond angle between atoms. Bond Angle

n Draw the Lewis Diagram. n Tally up e - pairs on central atom.  double/triple bonds = ONE pair n 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 LINEAR 180° BeH 2

3 total 3 bond 0 lone TRIGONAL PLANAR 120° BF 3 C. Common Molecular Shapes

3 total 2 bond 1 lone BENT <120° SO 2

4 total 4 bond 0 lone TETRAHEDRAL 109.5° CH 4 C. Common Molecular Shapes

4 total 3 bond 1 lone TRIGONAL PYRAMIDAL 107° NH 3 C. Common Molecular Shapes

4 total 2 bond 2 lone BENT 104.5° H2OH2O C. Common Molecular Shapes

5 total 5 bond 0 lone TRIGONAL BIPYRAMIDAL 120°/90° PCl 5 C. Common Molecular Shapes

6 total 6 bond 0 lone OCTAHEDRAL 90° SF 6 C. Common Molecular Shapes

n PF 3 D. Examples

n CO 2 D. Examples

IIIIII III. Molecular Polarity Molecular Structure

A. Dipole Moment n Direction of the polar bond in a molecule. n Arrow points toward the more e - neg atom. H Cl ++ --

B. Determining Molecular Polarity n Depends on:  dipole moments  molecular shape

B. Determining Molecular Polarity n Nonpolar Molecules  Dipole moments are symmetrical and cancel out. BF 3 F F F B

B. Determining Molecular Polarity n Polar Molecules  Dipole moments are asymmetrical and don’t cancel. net dipole moment H2OH2O H H O

CHCl 3 H Cl B. Determining Molecular Polarity n Therefore, polar molecules have...  asymmetrical shape (lone pairs) or  asymmetrical atoms net dipole moment

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