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1
Lewis structures give atomic connectivity
: they tell us which atoms are
physically connected to which, as well as types of covalent bonds
, number
of lone pairs
, formal charges
, resonance structures
Lewis structure does not provide the 3-dimentional shape of a molecule
The shape of a molecule is determined by its bond angles.
Consider CCl
4
: experimental Cl-C-Cl bond angles are 109.
Therefore, the molecule cannot be planar.
Molecular Shapes^ Molecular Shapes
Equilateral
triangle
tetrahedron vertex
C
Cl
Cl
Cl
Cl
In order to predict molecular shape, we assume the valence electron shellsof atoms in molecules repel each other.
Therefore, the molecule adopts 3D geometry that minimized this
repulsion.
We call this process Valence Shell Electron Pair Repulsion (VSEPR)
There are simple shapes for AB
2
and AB
3
molecules.
We can predict shapesusing VSEPR theory.
There are five fundamental geometries for molecular shape:
To determine the shape of a molecule, draw Lewis structure and find
(a) lone pairs (or non-bonding pairs) of electrons and
(b) bonding pairs (covalent bonds).
(c) all e-pairs (lone and bonding) are electron domains. E-domains repeleach other.
(d) 3D geometry: place in 3D space
ALL
electron domains, in a way to
minimize the e
-e
VSEPR Model repulsion
VSEPR Model
7
HNO
2
H
O
N
O
18e (9 pairs)
H
O
N
● ● O
H
O
N
● ● O
Double bond is
one domain
Central O: 4 domains, tetrahedralelectron domain structure Central N: 3 domains, trigonal planarelectron domain structure
Example:
Terminal O: 3 domains, trigonalplanar electron domain structure
H
O
N
O
Molecular geometry: ignore lone pairs
Central O:
electron domain structure: tetrahedral
Central O:
molecular structure: bent
Central N:
electron domain structure: trigonal planar
Central N:
molecular structure: bent
H
O
N
O
Molecular structure: trans-, cis-or gauche?
H
O
N
O
(trans- is 2.3 kJ/mol lower in energy than cis-)
The Effect of Nonbonding Electrons and
Multiple Bonds on Bond Angles
By experiment, the H-X-H bond angle decreases on moving from C to N toO:
Since electrons in a bond are attracted by two nuclei, they do not repelas much as lone pairs. Lone pairs occupy larger space.
Therefore, the bond angle decreases as the number of lone pairsincrease.
O
O
N
H
H
H
H^ C
H
H
H
O
O
H
H
11
The Effect of Nonbonding Electrons and Multiple Bonds
on Bond Angles
C
O
Cl Cl
o
o
Similarly, electrons in multiple bonds repel more than electrons in single bonds. Doublebond occupies larger space.
For octahedral structures, there is a plane containing fourelectron pairs. Similarly, the fifthand sixth electron pairs arelocated above and below thisplane.
Molecules with Expanded Valence Shells
To minimize e
-e
repulsion, lone pairs are always placed in equatorial
positions. (ClF
3
0
0
