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