Experiment 21 Lewis structures and VSEPR Theory
Introduction
1. Lewis Structures and Formal Charge
LG.N. Lewis, at the University of California at Berkeley devised a simple way to understand the
nature of the chemical bond in both ionic and molecular compounds. His method rests upon
focusing on the valence electrons, i.e. on the electrons in the outer most energy level of an atom
or ion. He represents these valence electrons as "dots" around the four sides of the elemental
symbol as shown in the figure below.
Lewis structures of molecules or ionic species are representations showing all electrons
(bonding and nonbonding)of the each element of the compound. The bonding or shared electron
pairs are shown either as lines or pair of dots between two atoms. The nonbonding or lone pairs
are shown as pair of dots on individual atoms. Atoms, other than hydrogen, tend to gain or lose,
or share electrons until it is surrounded by eight valence electrons satisfying the rule called
OCTET RULE. This is the electron arrangement corresponding to the configuration of a noble
gas (ns2np6). Here are the rules to write the Lewis structures for ionic or molecular compounds:
1. Draw skeletal structure of compound showing what atoms are bonded to each other.
2. Put the least electronegative element in the center (H and F are usually in terminal positions).
3. Count total number of valence e- and add 1 e- for each negative charge or subtract 1 e- for
each positive charge.
4. Draw single covalent bonds (2 electrons) between all the atoms and complete an octet for all
atoms except for the central atom.
5. Complete an octet for the central atom and, if the octet rule is not satisfied, add multiple
bonds on central atom as needed (see some examples below).
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Sometimes we can write more than one Lewis structure for certain molecules or polyatomic
ions. It is possible to determine the correct lewis structures using the concept of formal charge.
The formal charge of an atom is the charge that an atom (in a molecule) would have if all of the
atoms had the same electronegativity and is calculated using the following equation:
Formal Charge = (# of valence e-) – (# of nonbonding e-)– ½ (# of bonding e-)
So the best Lewis structure is the one with the fewest formal charges and that puts a negative
charge on the most electronegative atom. For example:
F. C. =
-2
0
+1
-2
0
+1
0 0 -1
best Lewis structure
2. VSEPR and Hybridization Theory
Lewis structure provides only the position of the atoms within the molecule, the number of
bonding and nonbonding electrons and the type of bonds. Valence Shell Electron Pair
Repulsion model (VSEPR) provides a method to predict the shape of most compounds. The
theory works on the principle that electron pairs in the valence shell repel each other and thus
will remain as far from each other as possible
The rules of this model are laid out in the table below, with E representing a cental atom and X
representing bonded atoms:
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Notice that the bonded atoms will attempt to fill the empty 3D space around a central atom
evenly. As lone pairs of electrons are introduced, the electron pairs will repel the atoms. If more
than one lone pair is present the lone pairs will repel each other first, and the overall shape of the
molecule is governed by the number of lone pairs. The bonded atoms will evenly fill up the
molecule wherever they may reach an equilibrium of repulsion between the lone pairs.
Let’s finally discuss how the the bonds between the atoms are formed so that the molecules can
assume the shape described in the VSEPR theory.
Hybridiation is a process in which atomic orbitals are mixed to form new identical orbitals and is
useful in the explanation of the shape of both ionic species and molecules. The concept of
hybridization is an integral part of valence bond theory. In the valence bond theory the bonds are
assumed to be formed by overlap of atomic orbitals and the shared region in space between
orbitals is the orbital overlap. There are two e- (one from each atom) with opposite spin in the
orbital overlap. Linus Pauling first developed the hybridization theory in order to explain the
structure of molecules such as methane (CH4). The hybrids are named based on the atomic
orbitals that are involved in the hybridization, and the geometries of the hybrids are also
reflective of those of the atomic-orbital contributors. For example, in the methane (CH4) a set of
sp3 orbitals are formed by mixing one s and three p orbitals on the carbon atom, and are directed
towards the four hydrogen atoms which are located at the vertices of a regular tetrahedron as
shown in the scheme below:
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Here are listed the steps necessary to predict the hybridization of the central atom
1. Draw a Lewis structure
2. Count the number of bonding pairs and the number of nonbonding pairs
3. Assign the electron-domain geometry using VSEPR theory
4. Specify the hybridization required to accommodate the bonding and nonbonding electron pairs
based on their geometric arrangement according to the following table:
Class
AB2
AB3
AB4
AB5
AB6
Hybridization
sp
sp2
sp3
sp3d
sp3d2
Electron Domain Geometry
linear
trigonal planar
tetrahedral
trigonal bipyramidal
octahedral
Where A represents the central atom and B the electron domains surrounding the central atom.
Procedure
Before attending the lab you will focus in assigning the Lewis structure and the formal charge of
the molecules and ions litsted in the table below, according to the rules described in the
introduction. During the lab period you will determine the VSEPR geometry of each species the
hybridization of the central atom, and the overall dipole moment and when possible its
orientation within the molecule or ion listed in the worksheet.
Visualizing these shapes is not always easy, and 3D models can help in this aspect. In the lab,
you will be gprovided with molecular modeling sets to build the molecules. The modeling
pieces are a useful tool because they will allow you to see whether or not the dgeometry you
have drawn will work within the VSEPR rules. Build a model of every molecule after you have
filled out it’s row in the table; if the model will not fit together this is a warning flag that the
geometry is incorrect. The 3D model will also be useful to help visualize 3D symmetry when
checking for a net dipole in the molecule. Also, building a 3D model will help you to interlalize
these visualizations, If you are having a hard time with the VSEPR diagram, try piecing together
a 3D model (or models) of what you have so far, and this may help you to see what comes next.
On each row in the worksheets, you will be given a compound. As you work through the
columns you will be guided toward the VSEPR diagram by working out the preliminary
information needed. Refer to the tables above as well as your book and lecture notes to draw the
diagram. Remember that this lab is to be done individually and all work must be your own.
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Worksheets:
Formula
and # of
assigned
valence
eletrons
Lewis
Structure
Central
atom(s)
(CAs)
Formal
charge
on each
atom
1. # of
lone pairs
2. # of
electron
domains
on CA(s)
Bond
angles
around
CA(s)
H2O
NH3
CH4
CO2
NH4+
H3O+
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1.Electron
domain
geometry
2.
Molecular
geometry
VSEPR structure
with approximately
correct bond angles
Dipole
moment
and
orientati
on
Hybrid.
of CA(s)
Formula
and # of
assigned
valence
eletrons
Lewis
Structure
Central
atom(s)
(CAs)
Formal
charge
on each
atom
1. # of
lone pairs
2. # of
electron
domains
on CA(s)
Bond
angles
around
CA(s)
N2H4
H2O2
HCN
CH3OH
CH3COOH
SO2
62
1.Electron
domain
geometry
2.
Molecular
geometry
VSEPR structure
with
approximately
correct bond
angles
Dipole
moment
and
orientation
Hybrid.
of CA(s)
Formula
and # of
assigned
valence
eletrons
Lewis
Structure
Central
atom(s)
(CAs)
Formal
charge
on each
atom
1. # of
lone pairs
2. # of
electron
domains
on CA(s)
Bond
angles
around
CA(s)
PCl5
SF4
ClF3
XeF2
SF6
ICl2-
63
1.Electron
domain
geometry
2.
Molecular
geometry
VSEPR structure
with approximately
correct bond angles
Dipole
moment
and
orientation
Hybrid.
of CA(s)
Formula
and # of
assigned
valence
eletrons
Lewis Structure
Central
atom(s)
(CAs)
Formal
charge
on
each
atom
1. # of
lone pairs
2. # of
electron
domains
on CA(s)
Bond
angles
around
CA(s)
BrF5
XeF4
H2SO4
SO42-
IF5
ICl4-
64
1.Electron
domain
geometry
2.
Molecular
geometry
VSEPR structure
with approximately
correct bond angles
Dipole
moment
and
orientation
Hybrid.
of CA(s)