10/29/2013
Sulfur Dioxide, SO2
1. Central atom = S
2. Valence electrons = 18 or 9 pairs
••
••
•
•
O
••
S
••
O
•
•
••
3. Form double bond so that S has an octet
— but note that there are two ways of doing
this.
OR bring in
right pair
bring in
left pair
••
••
•
•
O
••
S
••
O
•
•
••
Sulfur Dioxide, SO2
OR bring in
right pair
bring in
left pair
••
••
•
•
O
••
S
••
O
•
•
••
This leads to the following structures.
O S O
O S O
••
••
••
••
•
•
•
•
••
••
These equivalent structures are called
RESONANCE STRUCTURES. The true
electronic structure is a HYBRID of the two.
Sigma bonds result from a head-to-head overlap
of orbitals
Pi bonds result (only when there is a double or
triple bond) from a p-orbital side-to-side overlap
Urea, (NH2)2CO
1. Number of valence electrons = 24 ee2. Draw sigma bonds. (single first
O
bonds)
H
N
H
C
N
H
H
1
10/29/2013
3. Place remaining electron pairs in the
molecule.
O
••
•
•
•
•
••
H
N
••
C
H
N
H
H
4. Complete C atom octet with double bond.
(second bond is pi bond)
O
••
H
N
••
C
H
N
H
H
Exceptions to the Octet Rule
• Those nonmetals in period 3 or lower can
expand their octets (using d electrons) to
have 5 or even 6 pairs of electrons around
them when they are the central atom.
• Other exceptions are Boron ( 3 pairs) and
hydrogen (1 pair)
MOLECULAR GEOMETRY
VSEPR
• Valence Shell
Electron Pair
Repulsion theory.
Molecule
adopts the
shape that
minimizes the
electron pair
repulsions.
• Most important factor in
determining geometry is
relative repulsion
between electron pairs.
2
10/29/2013
There are 2 types of geometry that we are
concerned with…Electron-Pair geometry and
Molecular geometry.
Electron-Pair Geometry considers both LP and BP
from the central atom and can only be these 5
shapes.
Molecular Geometries –Usually
called the “shape”
• Molecular Geometries – include only the bonding
pairs.
• Choices include more than the five previous
mentioned..
• Linear = 1800 bond angle
• Bent (V-shaped)= 109.5 – 2(2.5) = 104.50 bond
angle
• Trigonal planar = 1200 bond angle
More Molecular Geometries
• Trigonal Pyramidal –unshared pair on central atom –
0
109.5 – 2.50= 107 bond angle
lone pair of electrons
in tetrahedral position
N
H
H
H
• Tetrahedral – no unshared pairs – 109.50 angle
3
10/29/2013
Geometries for
Four Electron Pairs
Molecular Geometries when more than
octet is on central atom
Molecular Geometries for Five Electron PairsPairs-all angles are
900 along one axis and 1200 along the other axis
Trigonal bipyramidal
Note what happens as unshared pairs are present,
only trigonal bipyramidal is an electron-pair
geometry
Molecular Geometries when more than
octet is on central atom
Molecular Geometries for Six
Electron Pairs
-all angles are 900
4
10/29/2013
# BP
# LP
Electron-Pair
Geometry
Molecular Geometry
2
0
Linear
Linear
3
0
Trigonal Planar
Trigonal Planar
4
0
Tetrahedral
Tetrahedral
5
0
Trigonal-bipyramidal
Trigonal-bipyramidal
6
0
Octahedral
Octahedral
1
3
Tetrahedral
Linear
2
2
Tetrahedral
Bent
3
1
Tetrahedral
Trigonal Pyramidal
# BP
# LP
Electron-Pair
Geometry
Molecular Geometry
5
0
Trigonal bipyramidal
Trigonal bipyramidal
4
1
Trigonal bipyramidal
Seesaw
3
2
Trigonal bipyramidal
T-shaped
2
3
Trigonal bipyramidal
Linear
6
0
Octahedral
Octahedral
5
1
Octahedral
Square pyramidal
4
2
Octahedral
Square planar
http://intro.chem.okstate.edu/1314F00/Lecture/Chapter10/VSEPR.html
Name the electron-pair and
molecular geometries…
Electron-Pair
1.
2.
3.
4.
5.
6.
7.
8.
Molecular
XeCl2
Trigonal bipyramidal
Linear
Octahedral
Square Pyramidal
ICl5
Tetrahedral
Bent
SCl2
Trigonal Planar
Bent
SeO2
PCl3
Tetrahedral
Trigonal Pyramidal
PCl5 Trigonal bipyramidal Trigonal bipyramidal
ICl3 Trigonal bipyramidal
T-shaped
H2CO Trigonal planar
Trigonal planar
5