Lecture 10 Molecular Geometry
Text Chapter 3, Sections 1,2,3
Molecular Shapes
ABn molecules
AB2
linear or bent
180°
A
B
A
B
B
linear
AB3
B
bent
trigonal planar, or trigonal pyramid, or (sometimes) T-shaped
120°
B
A
B
A
B
B
trigonal planar
B
B
B
A
B
B
trigonal pyramid
T-shaped
The VSEPR Method
Electron Domain (ED) – region in which it is most likely to find electrons. (This
name is not given in Atkins’ text but is commonly used.)
ED can contain three types of regions (illustrated on N atom in NO radical)
1) a single electron in a radical
2)bonding pairs – between two atoms
3)a nonbonding pair (commonly called lone pair) – located principally on one
atom.
Chapter 9 Molecular Geometry 1
1)The best arrangement of a given number of electron domains is the one that
minimizes the repulsions among them.
2)”Bond Regions” contain single, double or triple bonds
3a) The arrangement of electron domains about the central atom of an ABn
molecule is called its
electron-domain geometry (called in textbook, “regions of high electron
concentration”.
Number of Arrangement of
Electron
Predicted
electron
domain
bond
geometry
angles
linear
180°
trigonal
120°
electron domains
domains
2
B
A
B
3
B
4
A
planar
B
B
109.5° B
A
B
B
tetrahedral 109.5°
B
Chapter 9 Molecular Geometry 2
5
6
B
120° B
B A
B
90°
trigonal
120°
B
bipyramid
90°
B
B
B
A
B
B
B
octahedral 90°
180°
3b) The arrangement of the atoms (nuclei) of a molecule in space is the
“molecular geometry” (sometimes called “molecular shape”).
Steps to predict molecular geometries
1. Sketch the Lewis structure
2. Count the total number of electron domains around the central atom, and arrange
them in a way to minimize repulsions.
3. Describe the molecular geometry in terms of the angular arrangement of the
bonded atoms.
Chapter 9 Molecular Geometry 3
4. A double or triple bond is counted as one electron domain when predicting
geometry.
(# of electron domains) =
(# of atoms bonded to central atom) +
(# of nonbonding pairs on the central atom)
The effect of nonbonding electrons and multiple bonds on bond angles.
Electron domains for nonbonding electron pairs exert greater repulsive forces on
adjacent electron domains and thus tend to compress the bond angles.
H
C
H
H
H
109.5°
H
H
N
O
H
107°
H
H
104.5°
Electron domains for multiple bonds exert a greater repulsive force on adjacent
electron domains than do single bonds.
Chapter 9 Molecular Geometry 4
124.3°
Cl
C
111.4°
O
Cl
124.3°
“Normal” angle = 120o
Molecules with expanded valence shells.
Atoms from the third period and beyond can have more than four pairs of electrons
around them.
Five electron domains give rise to trigonal bipyramidal electron domain
geometries. Trigonal bipyramidal structures have three electron domains in the
equatorial position and two in the axial positions. Axials have three 90°
interactions while equatorial positions have only two. Put the (l.p.) bulkier domains
in the equatorial positions first.
Chapter 9 Molecular Geometry 5
Six electron domains give rise to octahedral geometries.
All electron domains are at 90° to four other electron domains.
Coordination Electronic
Number
Nonbonding Molecular Geometry
Geometry
Pairs
2
linear
0
linear
3
trigonal planar
0
trigonal planar
1
bent
0
tetrahedral
1
trigonal pyramid
2
bent
0
trigonal bipyramid
1
see-saw
2
T-shaped
3
linear
0
octahedral
1
square pyramid
2
square planar
4
5
6
tetrahedral
trigonal bipyramid
octahedral
Chapter 9 Molecular Geometry 6
trigonal
linear
trigonal bipyramid
tetrahedral
octahedral
F
O
C
N
O
O
O
O
Xe
H
H
F
F
F
B
F
F
N
H
H
F
Cl
H
F
F
H
Si
H
H
F
F
H
F
S
F
Xe
F
F
F
Cl
F
Cl
Cl
P
Cl
F
Br
F
F
F
Cl
F
F
F
S
F
F
F
Chapter 9 Molecular Geometry 7
Dr. Burke's Quick Method
(must first be familiar with valency of C,N,O atoms)
Number of El. Domains on atom =
Group No. + No. “pure” single bonds – No. triple bonds – charge
2
Number of LP on atom = No. ED – No. bond regions
Examples:
–
1) EDs on Sb atom in SbF4 =
5 + 4 - 0 - (-1)
=5
2
Trigonal bipyramid electronic geometry.
5 EDs - 4 bond regions = one lone pair.
F
F Sb F
F
F
3 x 90°
Sb F
F
F
2 x 90 + 2 x 120
Molecular geometry: see-saw (NOT trigonal pyramid shown on left)
2) EDs on S atom in SO2 =
(Remember that the valency on O atoms with only one partner is 2. So
double bond or bond in resonance.) LP = ED – Bond Regions = 3 – 2 = 1
Chapter 9 Molecular Geometry 8
3) EDs on S atom in SO32- =
LP = ED – Bond Regions = 4 – 3 = 1
4) EDs on S atom in SO42- =
LP = ED – Bond Regions = 4 – 4 = 0
5)`EDs on S atom in HSO4- =
LP = ED – Bond Regions = 4 – 4 = 0
Chapter 9 Molecular Geometry 9
Molecules with more than one central atom.
O
H
H
C
C
H
O
H
Polarity in Polyatomic Molecules
Chapter 9 Molecular Geometry 10
O
O
H
C
O
N
Cl
H
Cl
H
H
B
Cl
Cl
Cl
Cl
Cl
H
H
C
Cl
H
H
Dipole Moment = vector sum of bond moments
Chapter 9 Molecular Geometry 11
C
H
Cl