CHAPTER 7.2 MOLECULAR SHAPE

7.2
MOLECULAR SHAPE

LEARNING OUTCOMES

At the end of the lesson students should be
able to:
a) Explain Valence Shell Electron Pair Repulsion

Theory (VSEPR).
b) Draw the basic molecular shapes:

linear,
trigonal planar,
tetrahedral,
trigonal bipyramidal and
octahedral.
c) Predict and explain the shapes of molecule and

2

bond angles in a given species.

MOLECULAR SHAPE

➢ Lewis structures show the number and types of

bonds present in a molecule.

➢ However, they do not convey any information about

the molecular shape.

➢ Molecular shape is the three-dimensional

arrangement of the atoms within a molecule.

➢ It can be predicted by using Valence-Shell Electron

Pair Repulsion (VSEPR) theory. 3

VSEPR Theory

➢ The valence electron pairs on a central atom
repel each other and arrange themselves as
far apart as possible in order to minimize the
electron pair-electron pair repulsion.

➢ The order of the strength of repulsion between
electron pairs:

bonding pair lone pair lone pair
to
to < to <
bonding pair bonding pair lone pair
repulsion
repulsion repulsion
4

Applying VSEPR Theory

1. Determine the Lewis structure.
2. Determine the number of electron pairs around

the central atom. (Note: multiple bonds count as
SINGLE BOND)
3. Predict the arrangement of electron pairs using
VSEPR theory

5

Basic Molecular Shape

(Molecules In Which Central Atom Has No Lone Pair Electrons)

The specific arrangement adopted by the electron
pairs depends on the number of electron pairs
present.
o Two electron pairs – linear
o Three electron pairs – trigonal planar
o Four electron pairs – tetrahedral
o Five electron pairs – trigonal bipyramidal
o Six electron pairs - octahedral

6

(i) 2 bonding pairs - Linear structure

Class Number Number Shape Bond Angle
of bonded of lone
atom (B) pairs

B AB

AB2 2 0 180° 180o

Linear

(ii) 3 bonding pairs – Trigonal planar

Class Number Number Shape Bond Angle
of bonded of lone
atom (B) pairs

AB3 3 0 B 120o
A
BB

120°

(iii) 4 bonding pairs – Tetrahedral

Class Number Number Shape Bond Angle
of of lone
pairs
bonded
atom (B)

AB4 4 0 B 109.5o
AB
BB

109.5o

(iv) 5 bonding pairs – Trigonal bipyramidal

Class Number Number Shape Bond Angle
of of lone
pairs B 90o
bonded B and 120o
atom (B)
BA
AB5 5 0 B

B

90°

120°

(v) 6 bonding pairs – Octahedral

Number Number Shape Bond Angle
Class of bonded of lone
B 90o
atom (B) pairs BB

AB5 6 0 A
BBB

90°

90°

Drawing Basic Molecular Shape

1. Determine the Lewis structure.
2. Determine the number of electron pairs around the

CENTRAL ATOM – multiple bonds count as
SINGLE BOND.
3. Find out the appropriate VSEPR geometry for the
specified number of electron pairs, both bonding and
lone pairs.
4. Use the positions of atoms to establish the resulting
molecular geometry.

12

(i) Molecules with 2 bonding pairs

Example : BeCl2

Step 1: Lewis structure

No. of valence electron : •• ••
C••l Be C••l
Be = 2 e ••
••
2Cl = 14 e

Total = 16 e
– 4 e (bp)

12 e
– 12 e (lp)

0

13

•• ••
C••l Be C••l
Lewis Structure ••
••

No of electron pairs • 2 electron pairs
surrounding central • both are 2 bonding pairs
atom (Be)

Electron pair geometry Linear
@ basic geometry

Class AB2
VSPER
(electron pair repulsion) bonding pair-bonding pair electron
repulsion are equal
Molecular Shape
Cl Be Cl
Linear

Bond angle 180o 14

(ii) Molecules with 3 bonding pairs

Example : BCl3

Step 1: Lewis structure

No. of valence electron : F
B = 3e FB F
3 F = 21 e
Total = 24 e

– 6e
18 e

– 18 e (lp)
0

15

Lewis Structure F
FB F

No of electron pairs • 3 electron pairs
surrounding central • all are bonding pairs
atom

Electron pair geometry Trigonal planar
@ basic geometry

Class AB3

VSPER bonding pair-bonding pair electron

(electron pair repulsion) repulsion are equal

Molecular Geometry F Trigonal planar 16
Bond angle 120o
B
FF

(iii) Molecules with 4 bonding pairs
Example : CH4

◼ Lewis structure ◼ Class: AB4
◼ Shape:
H
H
H CH
109.5o
H
C

H H109.5o
H

Tetrahedral

17

(iv) Molecules with 5 bonding pairs

Example : PF5

◼ Lewis structure ◼ Class: AB5
◼ Shape:

F 90o F
F PF
F F PF

F F 120o

F

Trigonal bipyramidal

18

(v) Molecules with 6 bonding pairs

Example : SF6

◼ Lewis structure ◼ Class: AB6
◼ Shape:

F F F
FSF
FF 90o 90o

F FS F

19 F
F

Octahedral