Crescent School
SCH3U
Activity: Introduction to the 3-D Shape of Simple Molecules &
Polyatomic Ions
or
The Valence Shell Electron Pair Repulsion Theory—"Lite"
Rationale
The Valence Shell Electron Pair Repulsion Theory (VSEPR) is used to determine the 3-D
shape of molecules and polyatomic ions.
Required Background
You need to:
 understand covalent bonding—polar and non-polar
 be able to draw Lewis structures for simple molecules and polyatomic ions,
including molecules with double and triple bonds
Introduction
The premise of VSEPR theory: In a molecule or polyatomic ion, pairs of valence
electrons on the central atom (chemical bonds AND unbonded electron pairs) orient
themselves in such a way as to minimize electrostatic repulsion.
Example 1
Consider methane, CH4. The Lewis structure for methane shows 4 C–H bonds with no
lone pairs of electrons on C, the central atom.
fig 1. The Lewis structure for CH4.
This makes it look like methane is planar, with 90o bond angles. Not so.
Methane is a 3-D molecule, with a tetrahedral shape. Methane is symmetrical, with H-CH bond angles of 109.5o. The increased bond angles resulting from tetrahedral geometry
reduce repulsion between the pairs of bonding electrons.
fig 2. CH4 is three dimensional, with tetrahedral geometry. The bonding electron
pairs are at a maximum distance from one another in this shape.
Build a 3-D model of methane now. Take note of the bond angles. Save this model.
Example 2
Ammonia, NH3, is a little bit different. Its Lewis structure includes a lone (unbonded)
pair of electrons on N.
..
fig 3. Lewis structure for NH3.
Use the model of CH4 from the previous example. Remove one of the "bonds" and stand
this pyramid-shaped model on the benchtop. This is what a molecule of ammonia looks
like.
What effect does the lone pair on the central atom of NH3 have?
The lone pair of electrons on the central atom, N in this case, has the same spatial
requirements as a pair of electrons involved in a chemical bond. This means that the
overall, or electronic geometry of ammonia will also be tetrahedral. If we look only at
the chemical bonds—and not at the lone pair—we see that the actual, or molecular
geometry of NH3 is pyramidal, or pyramid-shaped.
fig 4. NH3 has pyramidal geometry. The lone pair on N (illustrated as lp) repels
the bonding electrons. Why do you think that the H-N-H bond angle is 107o
instead of 109.5o as in methane?
Let's make an important distinction: The electronic geometry of a molecule includes
bonding electrons and any lone pairs on the central atom. The molecular geometry is
based only on the arrangement of atoms. A molecule or polyatomic ion with no lone
pairs on the central atom will have a molecular geometry that is the same as its
electronic geometry. This is the case for CH4.
What is the electronic geometry of ammonia?
What is the molecular geometry of ammonia?
Example 3
Draw the Lewis structure for water, HOH, in the space below. (Writing the formula this
way emphasizes that O is the central atom.)
..
H: O : H
..
or H—O—H
..
..
fig 5. The Lewis structure of HOH.
How many lone pair(s) of electrons are on the central atom, O?
Adapt the molecular model of NH3 to get a model of HOH.
How would you describe the shape of a water molecule? No fancy name required.
Use the bond angles in CH4 and NH3 to predict the H–O–H bond angle in water. Provide
an explanation. Draw a pseudo 3-D diagram of a water molecule below.
fig 6. A pseudo 3-D diagram of a water molecule
Example 4
Draw the Lewis structure for SO2 in the space below. S is the central atom.
You should have one single coordinate covalent bond between S and O and one S=O
double bond. There should be a lone pair of electrons on S.
Notes:
1. The least electronegative atom will usually be in the centre. H, of course, can
never be in the centre. (Why?)
2. VSEPR theory treats double bonds and triple bonds the same as a single bond.
That is, the space requirements of a double and triple bond are similar to that of a
single bond.
What is the total of the chemical bonds and lone pairs?
How can three sets of electrons get as far from each other as possible? That is, what
geometry will this be?
Now assemble the corresponding molecular model.
Given that VSEPR theory treats double and triple bonds the same as single bonds, what is
the electronic geometry of SO2?
Take the lone pair of electrons on S into account: What is the molecular geometry of
SO2?
In this handout, we have limited our discussion to molecules and polyatomic ions that
have a maximum of (bonding pairs + lone pairs) = 4.
If you want, you can find out more about VSEPR theory at
http://www.molecules.org/VSEPR_table.html#top
There are many other websites devoted to VSEPR. Don't get bogged down with geeky,
university-level applications.
Table 1. Simple Molecular Shapes. A = central atom; X = bonding atom; E = lone pair on central atom
# bonds on
central atom
# lone pairs on
central atom
AX
1
0
AX2
2
0
AX3
3
0
AX2E
2
1
AX4
4
0
AX3E
3
1
AX2E2
2
2
formula
Lewis
structure
electronic
geometry
molecular
geometry
If there are no lone pairs of electrons on the central atom: electronic geometry = molecular geometry
diagram
Questions
IMPORTANT NOTE: FOR THE PURPOSES OF DETERMINING THE SHAPE OF A MOLECULE OR POLYATOMIC
ION, A DOUBLE OR TRIPLE BOND IS TREATED THE SAME AS A SINGLE BOND.
1. Use table 1 to help you complete the following table. Remember: the least electronegative element generally goes in the centre;
VSEPR theory treats double and triple bonds the same as single bonds.
formula
CH3Cl
PF3
CO2
HCN
ClO3–
H3O+
O3
Lewis
structure
# bonds on
central atom
# lone pairs on
central atom
electronic
geometry
molecular
geometry
diagram
2. Carbon is the central atom in phosgene, COCl2. Draw the Lewis structure and deduce the electronic and molecular geometry for
phosgene.
3. Boron trifluoride, BF3, is a stable molecule which contains three single B to F bonds. This means that B is “happy” with only 6
electrons in its valence shell. (This molecule does not follow the octet guideline.) Draw the Lewis structure and deduce the electronic
and molecular geometry for boron trifluoride.
—fin—