Exercise 12
Illinois Central College
CHEMISTRY 130
Laboratory Section: _______
Page 1
Name:___________________________
Molecular Structure
Objectives
To predict the shapes of molecules based on their Lewis Structures.
Background
The Valence Shell Electron Pair Repulsion Theory (VSEPR) is a model for predicting the shapes of
molecules and ions in which valence shell electron pairs are arranged around each atom so that electron
pairs are kept as far away from one another as possible, thus minimizing electron pair repulsions. For
example, if there are only two electron pairs in the valence shell of an atom, these pairs tend to be on
opposite sides of the nucleus. This gives a linear arrangement of electron pairs; that is, the electron pairs
mainly occupy regions of space at an angle of 180o to one another.
If three electron pairs are in the valence shell, they tend to be arranged in a plane toward the corners of
a triangle of equal sides. This arrangement is called trigonal planar, in which the regions of space
occupied by the electron pairs are directed at 120o angles to one another.
Four electron pairs in the valence shell tend to have a tetrahedral arrangement. The regions of space
mainly occupied by the electron pairs are directed at approximately 109.5o angles to one another.
In the event that an atom has an "expanded octet", that is, more than four pairs of electrons surrounding
it, two more possible shapes arise. Five electron pairs result in a trigonal bipyramid with three
"equatorial" pairs at 120o angles to one another and two "axial" positions at 90o angles to the "equator".
Six electron pairs results in an octahedral electron arrangement. In this case, each of the electron pairs
are directed at 90o angles to one another.
The Electron Pair Arrangements just discussed (See Figure 1.) can result in various "Molecular
Shapes" depending on how many of the electron pairs are actually used for bonding and how many
simply remain as "lone pairs" on the central atom of a molecule. The term 'Molecular Shape' refers to the
spatial arrangement of the bonded atoms only (As if the lone pairs on the central atom were simply
'invisible'.) These possible 'Molecular Shapes' for each of the five Electron Pair Arrangements are
summarized in Table 1.
Number of pairs
2
Linear
3
Trigonal planar
4
Tetrahedral
5
Trigonal bipyramid
Figure 1. Electron Pair Arrangements
6
Octahedral
Exercise 12 Page 2
Procedure
Fill in the Lewis Structures for all of the molecules indicated. SHOW ALL LONE PAIRS ON
CENTRAL ATOM AND PERIPHERAL ATOMS.
Construct models of all of the molecules or ions shown. HAVE YOUR INSTRUCTOR
CHECK YOUR MODELS ONCE YOU HAVE COMPLETED ALL OF THEM.
Refer to the color code on your model kit to determine the color used for each element. Note that
any trigonal bipyramid must use a brown sphere and octahedral models use a grey sphere.
Any lone pairs on the central atom should be represented with a vacant bonding peg as
they affect the Electron Pair Arrangement and ultimately, the Molecular Shape.
Use the longer flexible bonding pegs to construct any necessary double bonds.
Fill in all of the information for each column and for each molecule.
EXAMPLE:
Number of
electron
Lewis
density
Structure _ regions __
..
4
Species
NH3
H
N
Number of Electron Pair Molecular
lone pairs
Arrangement Shape
on central
(Sketch
(Sketch
Bond
atom
_ and Name) ___and Name) _ angle(s)
..
1
H
H
N
N
H
109.5o
H
H
H
tetrahedral
H
H
trigonal
pyramid
Exercise 12
ELECTRON PAIRS
Total
Bonding
Lone
ARRANGEMENT
OF PAIRS
MOLECULAR
SHAPE
Page 3
EXAMPLE
_________________________________________________________________________
2
2
0
Linear
Linear
3
3
0
Trigonal
planar
Trigonal
planar
2
1
4
0
3
1
Trigonal
pyramid
2
2
Bent (or
___________________________________________________________________________________________________________
Bent(or
angular)
_________________________________________________________________________
4
Tetrahedral
Tetrahedral
angular)
_________________________________________________________________________
5
5
0
Trigonal
bipyramid
Trigonal
bipyramid
4
1
Seesaw
3
2
T-shaped
2
3
Linear
_________________________________________________________________________
6
6
0
Octahedral
5
1
Square
pyramid
4
2
Square
planar
Table 1.
Octahedral
Exercise 12
Page 4
REPORT SHEET
Molecular Structure
Species
Lewis
Structure
The central atom is underlined
Number of
electron
density regions
Number of
lone pairs
on central atom
Electron pair
arrangement
(Sketch and Name)
Name:________________________
Molecular Shape
(Sketch and Name)
Bond
angle(s)
1. H2S
2. CO2
3. PH3
4. HCN
Exercise 12 Page 5
5. CF4
Species
6. SO3
7. XeF2
8. SF4
Exercise 12
Page 6
9. SbCl5
10. ICl5
Lewis
Structure
Number of
electron
density regions
Number of
lone pairs
on central atom
Electron pair
arrangement
(Sketch and Name)
Molecular Shape
(Sketch and Name)
Bond
angle(s)
Exercise 12
Illinois Central College
CHEMISTRY 130
Laboratory Section: ______
PRELAB: Exp.12
Page 7
Name:___________________________
Molecular Structure
1. Draw the Lewis Structures for SiF4, SF6, and ICl3. (The central atoms are underlined)
Show all of the lone pairs on the central atom and the peripheral atoms.
2. What are the Electron pair arrangements for each of the molecules in #1.
SiF4 ____________________
SF6 ___________________ ICl3 ____________________
3. What are the Molecular Shapes of the molecules in #1.
SiF4 ____________________
SF6 ___________________ ICl3 ____________________
4. Sketch the Molecular Shape for each of the molecules in #1. and indicate the bond angle(s).
Exercise 12
Page 8