Molecular Geometry
and
Bonding Theories
The properties of a molecule depend on its shape and
and the nature of its bonds. In this unit, we will
discuss three models.
(1) a model for the geometry of molecules
-- valence-shell electron-pair
repulsion (VSEPR) theory
(2) a model about WHY molecules form bonds
and WHY they have the shape they do
-- valence-bond theory
(3) a model of chemical bonding that deals with
the electronic structure of molecules
-- molecular orbital (MO) theory
bond angles: the angles made by the lines joining
the nuclei of a molecule’s atoms
carbon dioxide
CO2
180o
methane
CH4
109.5o
formaldehyde
CH2O
120o
VSEPR
electron domain: a region in which at least two
electrons are found
-- they repel each other because… they are all (–)
bonding domain: 2-to-6 e– that are shared by two
atoms; they form a… covalent bond
nonbonding domain: 2 e– that are located on a single
atom; also called a… lone pair
For ammonia, there are
NH3
..
three bonding domains and
H– N –H
one nonbonding domain.
..
H
N
H
Domains arrange themselves so
H
H
as to minimize their repulsions.
..
The electron-domain geometry is one
of five basic arrangements of domains.
N
H
H
H
-- it depends only on the total # of e– domains,
NOT the kind of each domain
The molecular geometry describes the
orientation of the atoms in space.
-- it depends on how many of each
kind of e– domain
..
Total # of Electron-Domain
Domains
Geometry
2
3
4
5
6
linear
Possible Molecular Geometries
linear (CO2)
trigonal
planar
trigonal planar (BF3), bent (NO2)
tetrahedral
tetrahedral (CH4),
trigonal pyramidal (NH3), bent (H2O)
trigonal
bipyramidal
trig. bipyramidal (PCl5), linear (XeF2)
seesaw (SF4), T-shaped (ClF3)
octahedral
octahedral (SF6), sq. pyr. (BrF5),
square planar (XeF4)
“atoms – axial”
To find the electron-domain geometry (EDG) and/or
molecular geometry (MG), draw the Lewis structure.
Multiple bonds count as a single domain.
Predict the EDG and MG of each of the following.
SeCl2
20 e–
..
..
.. .. ..
Cl–Se–Cl
.. .. ..
..
..
18 e–
]
–
..
O3
[
..
..
26 e–
..
SnCl3
–
.. .. ..
Cl–
.. Sn–Cl
..
Cl
..
..
O–
..
EDG: tetrahedral
MG: trig. pyramidal
EDG: trig. planar
MG: bent
EDG: tetrahedral
MG: bent
IF5
EDG: trig. planar
MG: trig. planar
..
..
..
..
..
..
..
..
34 e–
2–
]
F
..
..
SF4
[
..
24
e–
(res.)
CO32–
..
O– C= O
..
O
..
..
.. F ..
..F– S –F
..
..F
..
EDG: octahedral
MG: sq. pyramidal
I
42 e–
EDG: trig. bipyr.
MG: seesaw
28 e–
..
..
..
ClF3
..
..
..
..F
.. .. ..
..F– Cl –F
..
..F
EDG: trig. bipyr.
MG: T-shaped