Download NS 12.3 Geometry--VSEPR

NS 12.4
(pg 1 of 2)
Molecular Geometry (VSEPR)
Lewis structures help us understand the compositions of molecules and their covalent bonds. However, they do not tell us
anything about the three-dimensional shape of the molecule. Shape is an incredibly important aspect to determining the
behavior and function of all molecules. Molecules have shapes and sizes that are defined by the angles and distances
between the atoms that make up the molecule.
The shape and size of a molecule of a particular substance, together with the strength and polarity of its bonds, largely
determine the properties of that substance. Some of the most dramatic examples of the important roles of molecular
shape and size are seen in biochemical reactions. For example, a small change in the shape or size of a drug molecule may
enhance its effectiveness or reduce its side effects.
The sensations of smell and vision depend in part on molecular shape. When you inhale, molecules in the air are carried past
receptor sites in your nose. If the molecules have the right shape and size, they can fit properly on these receptor sites,
which transmit impulses to the brain. The brain then identifies these impulses as a particular aroma, such as the aroma of
freshly baked bread. The nose is so good at 3-D molecular recognition that two substances may produce different
sensations of odor even when their molecules differ as subtly as your right hand differs from your left.
First we must learn the relationship between two-dimensional Lewis structures and three-dimensional molecular shapes. The
lines that are used to depict bonds (single, double or triple) and non-bonded pairs in Lewis structures represent electron
domains that cause the shape of molecules. By examining these domains and the effect of bonding electrons compared with
nonbonding electrons we can gain a greater understanding of the shape of molecules. Later in this unit we will use this
geometrical information will assist in understanding the polarity of molecules, which dramatically affects the physical and
chemical properties of chemical compounds.
For example, the Lewis structure of CCl4 (on the left below) tells us only that four Cl atoms are bonded to a central C atom.
It does not tell us about the three-dimensional shape because the Lewis structure is drawn with the atoms in the same
plane. In the figure below, the actual three-dimensional arrangement of the atoms shows the Cl atoms at the corners of a
tetrahedron, a geometric object with four corners and four faces, each of which is an equilateral triangle.
Cl
Cl
Cl
C
Cl
Cl
109.5º
Cl
Cl
Cl
The overall shape of a molecule is determined by its bond angles, the angles made by the lines joining the nuclei of the
atoms in the molecule. The bond angles, of a molecule, together with the bond lengths accurately define the shape and size
of the molecule. In CH4 the bond angles are defined by moving along a bond from an H to the central C and then along
another bond to another H. All six H – C - H angles have the same value of 109.5°, which is characteristic of a tetrahedron.
In addition, all four C - H bonds are the same length.
In our discussion of the shapes of molecules we will begin with molecules (and ions) that, like CH 4 have a single central atom
bonded to two or more atoms.
NS 12.4 (pg 2 of 2)
Molecular Geometry
Using Balloons as a Model
Imagine tying two identical balloons together at their ends, the balloons naturally orient themselves to point away from
each other; that is, they try to "get out of each other's way" as much as possible. As a result, the balloons orient
themselves 180º away from each other. If we add a third balloon, the balloons orient themselves toward the vertices of an
equilateral triangle and are all 120º away from each other. If we add a fourth balloon, they adopt a tetrahedral shape and
will all be oriented the 109.5º away as mentioned above. There is an optimum geometry, therefore, for each number of
balloons.
In some ways the electrons in molecules behave like the balloons. We have seen that a single covalent bond is formed
between two atoms when a pair of electrons occupies the space between the atoms. We will refer to such a region as an
electron domain. Likewise, a nonbonding pair of electrons defines an electron domain that is located principally on one atom.
For example, the Lewis structure of NH3 has a total of four electron domains around the central nitrogen atom (three
bonding pairs and one nonbonding pair).
Each multiple bond in a molecule also constitutes a single electron domain. Thus, the structure for SO 2 has three electron
domains around the central sulfur atom; a single bond, a double bond, and a nonbonding pair of electrons.
Because electron domains are negatively charged, they repel one another. Therefore, like the balloons just discussed,
electron domains try to stay out of each other's way. The best arrangement of a given number of electron domains is the
one that minimizes the repulsions among those electron domains. This simple idea is the basis of the VSEPR model. VSEPR
stands for Valence Shell Electron Pair Repulsion. In fact, the analogy between electron domains and balloons is so close that
the same preferred geometries are found in both cases. Thus, like the balloons mentioned, two electron domains are
arranged linearly, three domains are arranged in a trigonal-planar fashion, and four are arranged tetrahedrally. These
arrangements will be summarized and built in class with various model kits and are summarized in the chart below.
electron
domains
4
electron domain
geometry
tetrahedral
bonding
domains
4
3
nonbonding
domains
0
1
2
2
bent
3
0
1
linear
trigonal planar
bent
0
linear
3
trigonal planar
1
3
2
2
linear
2
molecular geometry
bond angles
example
tetrahedral
trigonal pyramid
109.5º
< 109.5º
(~107º)
<< 109.5º
(~104.7º)
180º
120º
< 120º
(~116º)
180º
CCl4
PF3
H 2S
HI
NO31O3
SiS2
The arrangement of electron domains about the central atom in a molecule or ion is called its electron-domain geometry.
(the shape of the electrons). The arrangement of the atoms is called the molecular geometry (the shape of the molecule).
In the VSEPR model, we predict the molecular geometry of a molecule or ion from inspecting its electron-domain geometry
and then considering the number of unshared pairs of electrons which will affect the shape.
To predict the shapes of molecules with the VSEPR model, we use the following steps:
1
Draw the Lewis structure of the molecule or ion, and count the total number of electron domains around the
central atom. Each nonbonding electron pair, each single bond, each double bond, and each triple bond counts as
an electron domain.
2 Determine the electron-domain geometry by arranging the total number of electron domains so that the
repulsions among them are minimized in other words, spread out the domains as much as possible.
3 Use the arrangement of the bonded atoms to name the molecular geometry.