Section 3.3 – Part A
Pg. 91-96
Objective:
1) Apply VSEPR theory to predict molecular shapes

Stereochemistry – is the study of the 3-D spatial
configuration of molecules and how this affects
their reactions.
Solid = in plane of page Dashed = behind (away) Wedge = ahead (toward)


The shape of molecules is determined by the repulsion that
happens between electron pairs
The theory behind molecular shapes is called VSEPR Theory
(Valence Shell Electron Pair Repulsion)

General Rule:
◦ Pairs of electrons in the valence shell of an atom stay
as far apart as possible because of the repulsion of
their negative charges
◦ The type, number and direction of bonds to the
central atom of a molecule determine the shape of
the resulting molecule.

So how do we predict these molecular shapes?

We will be using the following compounds to
analyze the 6 shapes possible
◦ BeH2(s), BH3(g), CH4(g), NH3(g), H2O(l), HF(g)

To start, draw a Lewis formula for each of the
molecules and then consider the arrangement of all
pairs of valence electrons.
◦ (Remember – all pairs of valence e-’s repel each
other and want to get as far apart as possible)
Lewis
Formula
Bond
Pairs
Be
2
*
Lone Total General
Pairs Pairs Formula
0
2
Electron Pair
Arrangement
Stereochemical
Formula
linear
X–A–X
linear
AX2
A is the central atom; X is another atom
• This Lewis formula indicates that BeH2(s) has two bonds and
no lone pairs on the central atom.
• VSPER theory suggests that the two bond pairs will be farthest
apart by moving to opposite sides to a bond angle of 180°
• This gives the molecule a linear orientation
*Exception* Beryllium does not follow OCTET RULE
Lewis
Formula
Bond
Pairs
3
B
*
Lone Total General
Pairs Pairs Formula
0
3
AX3
Electron Pair
Arrangement
Stereochemical
Formula
trigonal
planar
A is the central atom; X is another atom
• This Lewis formula indicates that BH3(g) has three bonds and
no lone pairs on the central atom.
• VSPER theory suggests that the three bond pairs will be farthest
apart by moving to a bond angle of 120° to each other.
• This gives the molecule a trigonal planar orientation.
*Exception* - Boron Does not follow OCTET RULE

Draw the Lewis Formula for BF3
Does not obey the octet rule
F
Trigonal Planar
F
F
Lewis
Formula
Bond
Pairs
4
*
Lone Total General
Pairs Pairs Formula
0
4
Electron Pair
Arrangement
AX4 tetrahedral
A is the central atom; X is another atom
• This Lewis formula indicates that CH4(g) has four bonds
and no lone pairs on the central atom.
• VSPER theory suggests that the four bond pairs will be
farthest apart by arranging in three dimensions so that
every bond makes an angle of 109.5° with each other.
• This gives the molecule a tetrahedral orientation.
Stereochemical
Formula

Draw the Lewis Formula for SiH4
H
Tetrahedral
H
H
H
Lewis
Formula
Bond
Pairs
3
Lone Total General
Pairs Pairs Formula
1
4
AX3E
Electron Pair
Arrangement
Stereochemical
Formula
tetrahedral
Trigonal pyramidal
*
A is the central atom; X is another atom, E is a lone pair of electrons
• This Lewis formula indicates that NH3(g) has three bonds and
one lone pair on the central atom.
• VSPER theory suggests that the four groups of e-’s should repel
each other to form a tetrahedral shape (bond angle = 109.5°)
• But the lone pair is very repulsive, thus pushes the atoms more
to a 107.3° bond angle
• This gives the molecule a trigonal pyramidal orientation.

Draw the Lewis Formula for PCl3
Cl
Cl
Cl
Trigonal pyramidal
Lewis
Formula
Bond
Pairs
2
*
Lone Total General
Pairs Pairs Formula
2
4
AX2E2
Electron Pair
Arrangement
Stereochemical
Formula
tetrahedral
Angular
(Bent)
A is the central atom; X is another atom, E is a lone pair of electrons
• This Lewis formula indicates that H2O(l) has two bonds and
two lone pairs on the central atom.
• VSPER theory suggests that the four groups of e-’s should repel
each other to form a tetrahedral shape (bond angle = 109.5°)
• But the TWO lone pairs are very repulsive, thus pushes the
atoms more to a 105° bond angle
• This gives the molecule an angular (bent) orientation.

Draw the Lewis Formula for OCl2
Angular (bent)
Lewis
Formula
H
*
F
Bond
Pairs
1
Lone Total General
Pairs Pairs Formula
3
4
AXE3
Electron Pair
Arrangement
Stereochemical
Formula
Linear
(Tetrahedral)
A is the central atom; X is another atom, E is a lone pair of electrons
• This Lewis formula indicates that H2O(l) has two bonds and
two lone pairs on the central atom.
• VSPER theory suggests that the four groups of e-’s should repel
each other to form a tetrahedral shape (bond angle = 109.5°)
• But since there are only two atoms with one covalent bond
holding them together, by definition, the shape is linear, as is the
shape of every other diatomic molecule.

Draw the Lewis Formula for HCl

VSEPR theory describes, explains, and predicts the
geometry of molecules by counting pairs of electrons that
repel each other to minimize repulsion. The process for
predicting the shape of a molecule is summarized below:

Step 1: Draw the Lewis formula for the molecule, including
the electron pairs around the central atom.

Step 2: Count the total number of bonding pairs (bonded
atoms) and lone pairs of electrons around the central atom.

Step 3: Refer to Table 7, and use the number of pairs of
electrons to predict the shape of the molecule.
Pg. 95

Draw the Lewis and stereochemical formulas
for a sulfate ion, SO42- and predict the shape
◦ See pg. 95

Draw the Lewis and stereochemical formulas
for a chlorate ion, ClO3- and predict the shape
◦ See pg. 96

On your own: Pg. 96 #3


It is important to remember that a double or triple bond is one
bond, and to treat it as such, when predicting the VSEPR shapes of
molecules.
Example: Predict the shape of C2H4(g)
◦ Draw the Lewis formula for the molecule
H H
H
H
◦ Count the # of pairs of e-’s around the central carbon atoms.
 The carbon atoms have 3 bonds (2 single, 1 double) and no
lone pairs.
 This is the same as a trigonal planar configuration.

Practice: Predict the shape for C2H2(g).
Answer: See pg. 97
1)
Finish pg. 96 #1-3
2)
Pg. 98 #6-7 (Multiple Bond Practice)
◦ For 7 c, d, e - If there is more than one central atom involved,
tell me the shape around each of the central atoms
◦ Example:
trigonal planar—first two carbons
tetrahedral—third carbon
3)
Pg. 104 #1, 2, 3
 #2: If there is more than one central atom involved, tell me
the shape around each of the central atoms

Draw the Lewis Formula for PCl3