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121 | P a g e
EXPERIMENT #13
Lewis Structures and Molecular Geometry
OBJECTIVES:
Draw Lewis structures of atoms, ions, and molecules
Build models of linear, trigonal planar tetrahedral, trigonal bipyramidal, and octahedral
arrangements of electron pairs
Relate proper Lewis structure with the correct three dimensional structure of the molecule or ion
Even before physicists and chemists understood the quantum nature of the electron and the
chemical bond, G. N. Lewis observed that the valence electrons of stable molecules and ions are
arranged in four pairs for a total of eight an octet of electrons. The Lewis structure of a molecule or
ion emphasizes this observation. Since electrons are negatively charged and negative charges repel
each other, we can assume these repulsions can be minimized by keeping the electrons in bonds as far
from each other as possible. Sidgwick and Powell and more recently Gillespie and Nyholm have
refined this assumption and have shown that well defined geometric shapes are the result. These shapes
maximize distances (minimize electron repulsions) in three dimensional space. It is these shapes that a
molecule or ion may take. Thus, a knowledge of the Lewis structure of a molecule allows us to make
predictions about its shape. Once the shape of a molecule is known, properties dependent upon its
shape, such as bond hybridization and dipole moment, can be predicted.
The central atom in a molecule or ion may have two, three, four, five, or six electron domains
(or groups) surrounding it. These electron domains may form bonds with other atoms or they may be
non-bonding domains. A domain may consist of a single pair of electrons (single bond or non-bonding
pair), two pairs (double bond) or three pairs (triple bond). For example, in the Lewis structure for
ammonia, NH3, the central nitrogen atom is surrounded by four electron domains. Three of these
domains are single bonds to hydrogen atoms, the remaining electron domain is the lone pair or non-
bonding pair. The orientation of the bonds around the central atom, in other words the three
dimensional shape, is determined by the number of electron domains surrounding the central atom.
The chart below describes the electron domain geometry around the central atom. When using this
chart, it is important to remember that multiple bonds (double and triple bonds) are considered as a
single electron domain.
non-bonding or lone pairs
bonding pairs
The shape of the molecule or ion is related to the electron domain geometry. The name given to
the shape of a molecule or ion is the shape defined by the number of bonding domains. This is
summarized in the chart below. When using this chart, it is important to remember that multiple bonds
(double and triple bonds) are considered as a single electron domain.)
H N:
H
H
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EXPERIMENT #13 LEWIS STRUCTURES
TABLE I: ELECTRON DOMAIN GEOMETRY
Number of Electron
Domains Around the
Central Atom
Electron Domain
Geometry
Predicted Bond Angles Hybridization
2 linear 180° sp
3 trigonal planar 120° sp2
4 tetrahedral 109.5° sp3
5 trigonal bipyramidal 120°
90°
sp3d
6 octahedral 90° sp3d2
TABLE II: MOLECULAR GEOMETRY
Number of Electron
Domains Surrounding
the Central Atom
Number of Bonding
Domains Surrounding
the Central Atom
Molecular Geometry Example
2 2
linear CO2
3
3
trigonal planar
BF3
2 angular, planar bent, V-shaped
(120° bond angle) NO2
-
4 4 tetrahedral CH4
3 pyramidal NH3
2 angular, planar bent, V-shaped
(109.5° bond angle)
H2O
5 5 trigonal bipyramidal PCl5
4 seesaw SF4
3 T-shaped ClF3
2 linear XeF2
6 6 octahedral SF6
5 square pyramidal BrF5
4 square planar XeF4
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EXPERIMENT #13 LEWIS STRUCTURES
Linear Trigonal V-Shape, 120
Tetrahedral Pyramidal V-Shape, 109.5
Trigonal Bipyramid See-Saw T-Shape Linear
(Distorted Tetrahedron)
Octahedron Square Pyramid Square Planar
FIGURE I: Molecular Shapes Derived from Solid Geometry
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EXPERIMENT #13 LEWIS STRUCTURES
Rules for Drawing Lewis Structures
1. Count up the valence electrons for all atoms in formula.
2. Add an electron for each negative charge on an ion; subtract an electron for each positive charge on an ion.
3. Attach atoms in a way which makes “chemical sense.” Draw a line between each attached pair; this consumes
two electrons from the total calculated in step 2.
4. Add pairs of electrons to fulfill the octet rule; duet rule for hydrogen.
5. If necessary make nonbonding pair(s) bonding pair(s), but do not alter the total number of electrons calculated
in step 2.
6. If necessary, expand octet for atoms in third row or lower in the periodic table.
PROCEDURE: (Review the rules for drawing Lewis structures before coming to lab.)
1. From the group of molecules/ions you are assigned, draw the Lewis structures using the rules you
learned in class. This will allow you to determine the geometry around the central atom.
2. From the model kits find the spheres having 4, 5, and 6 holes drilled in them. These spheres will
represent a central atom in a molecule or ion. The number of holes drilled into the ball will be
usedto represent the number of electron domains around the central atom.
3. Using the sphere containing five holes, find the holes which are diametrically opposed. (These
holes are 180° apart.) Place a wooden stick into each of these two holes. A molecule or ion
containing a central atom surrounded by only two electron domains will have a linear arrangement
of these electron domains. If these are bonding domains, the molecule will be linear; that is, it will
have the shape of a straight line.
4. Using the sphere containing five holes, find the three holes in the same plane. (These holes are
120°apart.) Place a wooden stick into each of these three holes. A molecule or ion containing a
central atom surrounded by three electron domains will have a trigonal planar arrangement of
these electron domains. If these are bonding domains, the molecule will be trigonally planar; that
is, it will have the shape of a triangle. (The triangle will be an equilateral triangle, if the three
bonds are identical.)
5. Remove one of the wooden sticks. This is a model of a molecule or ion whose central atom is
surrounded by three electron domains, but contains only two bonding domains. This molecule is
V-shaped. It is a planar molecule with a bond angle of 120°.
6. Using the sphere containing four holes, build a model of a tetrahedron by placing a wooden stick
into each hole. The angle between each of the sticks is 109°. It is important to be able to draw a
three dimensional tetrahedron on a two dimensional piece of paper. Practice drawing a
tetrahedron. Watch closely as your instructor demonstrates how to do this. [The solid line
(FIGURE I) represents a bond or electron group in the plane of the paper. For a tetrahedron there
are two such lines. The wedge indicates a bond or electron domain coming out of the plane of the
page toward the reader, while the dashed line indicates a bond or electron domain going behind
the plane of the paper, away from the reader.]
125 | P a g e
EXPERIMENT #13 LEWIS STRUCTURES
7. Remove one stick from the tetrahedron. The resulting three dimensional shape is a pyramid. The
angle between each electron domain of the pyramid is approximately 109°. If you can draw a
tetrahedron, you can also draw a pyramid.
8. Remove a second stick from the tetrahedron. The resulting three dimensional shape is a planar,
bent shape. It is also called V-shaped, but the angle between the two bonds is approximately 109°
(see above).
9. Go back to the sphere containing five holes. Place a wooden stick into each of the five holes. A
molecule or ion containing central atom surrounded by five electron domains will have a trigonal
bipyramidal arrangement of these electron domains. If these are all bonding domains, the molecule
will be a trigonal bipyramid. Notice that there are two types of electron domains, de- pending on
their orientation in space. The three electron domains 120° from each other are in the same plane.
They form a triangle and are said to occupy the equatorial positions of a trigonal bipyramid. The
two electron domains 180° from each other are said to occupy the axial positions of a trigonal
bipyramid. Watch closely as your instructor demonstrates how to draw a trigonal bipyramid. The
solid (FIGURE I) line represents a bond or electron group in the plane of the paper. Note that
there are three such lines. The wedge indicates a bond or electron domain coming out of the plane
of the page, while the dashed line indicates a bond or electron domain going behind the plane of
the paper.]
10. The shape of a molecule or ion having a central atom surrounded by five electron domains, but
containing only four bonds, is made from the trigonal bipyramid by removing one of the wooden
sticks. But which stick do we remove, the axial or the equatorial? It turns out that removal of one
of the equatorial sticks (or electron domains) gives the shape having the minimum amount of
electron domain repulsions. (You may want to convince yourself of this by discussions with your
classmates or your instructor.) The shape of a molecules whose central atom is surrounded by five
electron groups, but contains only four bonds is a distorted tetrahedron or see-saw. Build such a
model by removing an equatorial bond from your model of a trigonal bipyramid. The bond angles
are 180°, 120°, and 90°.
11. If we remove one more equatorial bond, we see the shape of a molecule whose central atom is
surrounded by five electron domains, but contains only three bonds. Such a molecule is T-shaped.
The bond angles are 180° and 90°.
12. Using the sphere containing six holes, build a model of an octahedron by placing a wooden s tick
into each hole. The angle between each of the sticks is 90°. It is important to be able to draw a
three dimensional octahedron on a two dimensional piece of paper. Practice drawing an
octahedron. Watch closely as your instructor demonstrates how to do this. The solid line
(FIGURE I) represents a bond or electron domain in the plane of the paper. For an octahedron
there are two such lines. The wedge indicates a bond or electron domain coming out of the plane
126 | P a g e
EXPERIMENT #13 LEWIS STRUCTURES
of the page; there are two such lines. The dashed line indicates a bond or electron domain going
behind the plane of the paper; there are two such lines. If all the atoms bonded to the central atom
in the octahedron are the same, then each of the bonds in the octahedron is identical. Build such a
model to show that this statement is true. When drawn as shown in FIGURE I, an octahedron
appears to have two types of bonds. The four bonds forming the square are equatorial bonds, and
the bond above the square and the bond below the square are axial bonds.
13. The shape of a molecule or ion having a central atom surrounded by six electron domains, but
containing only five bonds, is made from the octahedron by removing one of the wooden sticks.
But which stick do we remove, the axial or the equatorial? It turns out that removal of one of the
axial sticks (or bonds) gives the shape having the minimum amount of electronic repulsions. (You
may want to convince yourself of this by discussions with your classmates or your instructor.) The
shape of a molecule whose central atom is surrounded by six electron domains, but contains only
five bonds is a square pyramid. The bond angles are 90°. Build such a model by removing an axial
bond from your model of an octahedron.
14. If we remove one more axial bond, we see the shape of a molecule whose central atom is
surrounded by six electron domains, but contains only four bonds. Such a molecules is a square
plane. The bond angles are 90°.
127 | P a g e
NAME________________________________ Section_______ Date__________
Lewis Structures and Molecular Geometry
In the boxes provided below draw the Lewis structure of each ion or molecule from the groups
you are assigned. Also, include in the space indicated, the formula, the number of valence electrons,
electron domain geometry, number of bonds, molecular geometry, and the hybridization of the central
atom.
Group I:
CO2, BCl3, SO3, CHCl3, SO42, H2S, NI3, SO2, PCl5, SF4, ClF3, I3
, C2H2, SF6, BrF5, XeF4
Group III:
HCN, BH3, CO32, CHFCl2, PO4
3, OF2, PCl3, PbCl2, SOF4, IF4+, IF3, IF2
, C2H2Cl2, XeO64, IF5, IF2Br2
Group IV:
BeCl2, AlBr3, CCl4, XeO4, SF2, XeO3, PH3, SnBr2, SbCl5, IO2F2, ICl2
, C2F2Cl2, TeF6, SF5, XeOF4, XeBr4
Group V H2CCCH2, C2H2Cl2, H2CO, CH3C(=O)OH, C6H6 (all carbons sp2) Lewis Structure
Molecule
Number of Valence Electrons
Electron Group Geometry
Number of Bonding Domains
Molecular Geometry
Hybridization of Central Atom
Lewis Structure
Molecule
Number of Valence Electrons
Electron Group Geometry
Number of Bonding Domains
Molecular Geometry
Hybridization of Central Atom
128 | P a g e
Lewis Structure
Molecule
Number of Valence Electrons
Electron Group Geometry
Number of Bonding Domains
Molecular Geometry
Hybridization of Central Atom
Lewis Structure
Molecule
Number of Valence Electrons
Electron Group Geometry
Number of Bonding Domains
Molecular Geometry
Hybridization of Central Atom
Lewis Structure
Molecule
Number of Valence Electrons
Electron Group Geometry
Number of Bonding Domains
Molecular Geometry
Hybridization of Central Atom
129 | P a g e
NAME________________________________ Section_______ Date__________
Lewis Structure
Molecule
Number of Valence Electrons
Electron Group Geometry
Number of Bonding Domains
Molecular Geometry
Hybridization of Central Atom
Lewis Structure
Molecule
Number of Valence Electrons
Electron Group Geometry
Number of Bonding Domains
Molecular Geometry
Hybridization of Central Atom
Lewis Structure
Molecule
Number of Valence Electrons
Electron Group Geometry
Number of Bonding Domains
Molecular Geometry
Hybridization of Central Atom
130 | P a g e
Lewis Structure
Molecule
Number of Valence Electrons
Electron Group Geometry
Number of Bonding Domains
Molecular Geometry
Hybridization of Central Atom
Lewis Structure
Molecule
Number of Valence Electrons
Electron Group Geometry
Number of Bonding Domains
Molecular Geometry
Hybridization of Central Atom
Lewis Structure
Molecule
Number of Valence Electrons
Electron Group Geometry
Number of Bonding Domains
Molecular Geometry
Hybridization of Central Atom
131 | P a g e
NAME________________________________ Section_______ Date__________
Lewis Structure
Molecule
Number of Valence Electrons
Electron Group Geometry
Number of Bonding Domains
Molecular Geometry
Hybridization of Central Atom
Lewis Structure
Molecule
Number of Valence Electrons
Electron Group Geometry
Number of Bonding Domains
Molecular Geometry
Hybridization of Central Atom
Lewis Structure
Molecule
Number of Valence Electrons
Electron Group Geometry
Number of Bonding Domains
Molecular Geometry
Hybridization of Central Atom
132 | P a g e
Lewis Structure
Molecule
Number of Valence Electrons
Electron Group Geometry
Number of Bonding Domains
Molecular Geometry
Hybridization of Central Atom
Lewis Structure
Molecule
Number of Valence Electrons
Electron Group Geometry
Number of Bonding Domains
Molecular Geometry
Hybridization of Central Atom
Lewis Structure
Molecule
Number of Valence Electrons
Electron Group Geometry
Number of Bonding Domains
Molecular Geometry
Hybridization of Central Atom
133 | P a g e
NAME________________________________ Section_______ Date__________
Lewis Structure
Molecule
Number of Valence Electrons
Electron Group Geometry
Number of Bonding Domains
Molecular Geometry
Hybridization of Central Atom
Lewis Structure
Molecule
Number of Valence Electrons
Electron Group Geometry
Number of Bonding Domains
Molecular Geometry
Hybridization of Central Atom
Lewis Structure
Molecule
Number of Valence Electrons
Electron Group Geometry
Number of Bonding Domains
Molecular Geometry
Hybridization of Central Atom
134 | P a g e
Lewis Structure
Molecule
Number of Valence Electrons
Electron Group Geometry
Number of Bonding Domains
Molecular Geometry
Hybridization of Central Atom
Lewis Structure
Molecule
Number of Valence Electrons
Electron Group Geometry
Number of Bonding Domains
Molecular Geometry
Hybridization of Central Atom
Lewis Structure
Molecule
Number of Valence Electrons
Electron Group Geometry
Number of Bonding Domains
Molecular Geometry
Hybridization of Central Atom