4.2 MOLECULAR GEOMETRY

4.2 Molecular Shape and Polarity

What is VSEPR?

“Valence Shell Electron Pair Repulsion”

The theory states;
“ The valence electron pairs surrounding a central

atom repel one another. Bonding pairs and electron
pairs are arrange to be as far apart as possible”.

Predict the molecular geometry of a molecule.

TYPES OF ATOM

●● Central atom

●●I ●●

Central atom ●● ●● Terminal atom

●●●I●—●N●—●I●●●

Terminal atom

••O•• • ••O••••

N

TYPES OF ELECTRON

Consider electrons ONLY around central atom

Lone electron ●● Bonding pair electron
Lone pair electron
●●I ●●

●● ●●

●●●I●—●N●—●I●●●

Bonding pair electron

••O•• • ••O••••

N

REMEMBER!!!

lone electron = Lone pair electron

Double bonds and triple bonds are treated

as single bond (approximation)
 one bond pair electron

Order of electron pairs repulsion:

The order of the repulsion strength of electron pairs,

Lone pair-lone pair > Lone pair-bond pair > Bond pair-bond pair
repulsion repulsion repulsion

O
HH

FIVE BASIC MOLECULAR SHAPES

Molecular geometry Shape Angle

Linear 180o

Trigonal planar 120o

Tetrahedral 109.5o

Trigonal bipyramidal 90o , 120o

Octahedral 90o

ANGLE OF FIVE BASIC MOLECULAR SHAPES

Linear (180o)

Trigonal planar (120o)
Tetrahedral (109.5o)

Trigonal bipyramidal (90o , 120o)

terminal atoms Octahedral (90o)
central atom

BASIC MOLECULAR SHAPE:
Central atom without lone pair electron

General No. of atoms No. lone Arrangement Molecular Shape
formula bonded to pairs on of electron Geometry
central pairs
central atom
atom

AB2 2 0 linear linear
AB3 3
AB4 4 BB

0 Trigonal Trigonal

planar planar

0 Tetrahedral Tetrahedral

AB5 5 0 Trigonal Trigonal
AB6 6
bipyramidal bipyramidal

0 octahedral octahedral

COVALENT BOND

chemical bond that involves the sharing of electron pairs
between atoms.
covalent bond between two atom in a molecule can be
classified as:

I. Non polar bond
II. Polar bond

NON POLAR BOND

A bond between atoms of same electronegativity
in which the distribution of the density of the bonding
electron pair is symmetrical.

Cl  Cl

Non polar bond

POLAR BOND

A bond between atoms of different electronegativity
in which the distribution of the density of the bonding
electron pair is asymmetrical.

The direction of polarity of a polar bond
can be symbolised by a vector quantity

The crossed end of the arrow is the
positive end and the arrow head is
the negative end.

+ -

H  Cl

Cross end Arrow head

EXAMPLE –03

a) Use a polar arrow to indicate the polarity of
each bond: N–H, F–N, I–Cl

b) Rank the following bonds in order of
increasing polarity: H–N, H–O, H–C

Refer to book for the value of
electronegativity of the elements
(if necessary).

Electronegativity: N;3.0
H;2.1 Cl;3.0
C;2.5 I;2.5
F;4.0
O;3.5

Ans: EXAMPLE – 03

a) indicate the polarity

N—H F—N I—Cl

b) Increasing order of bond polarity
H–C < H–N < H–O

EXAMPLE – 04

Arrange each set of bonds in order of
increasing polarity, and indicate bond polarity
with + and – symbols:
a) Cl–F, Br–Cl, Cl–Cl
b) Si–Cl, , Si–F, Si–Si

Electronegativity:
Br;2.8
Cl;3.0
F;4.0
Si;1.8

Ans: EXAMPLE – 04

a) Increasing order of bond polarity :

+ – + –
Cl—Cl < Br—Cl < Cl—F

b) Increasing order of bond polarity :

+ – + –
Si—Si < Si—Cl < Si—F

POLAR MOLECULE

A polar molecule contain polar covalent bond(s) and
it has non-zero dipole moment (µ=0).

A dipole molecule in which the positive and negative
pole can be distinguished
(because of the separation of the charge).

electron poor electron rich
region region

H F

 

Determine polarity of H2O?

When a negatively charged rod is brought close to a
thin stream of water, deflection occurs.

O
H H ≠0

H2O example of a polar molecule

NON-POLAR MOLECULES

 Covalent bonds between atoms of the same element

 The atoms have no difference in electronegativity.
 Bonding electrons are shared equally.

 Form a non-polar molecule.

Example: : F─F
: O─O
F2 : N─N
O2
N2

A quantitative measure of molecular polarity
=Qxr
Q: charge
r: distance between charges
1 D (Debye) = 3.36 x 10-30 C m

RESULTANT (NET) DIPOLE MOMENT

Determined by molecular shape and bond polarity
Resultant dipole moment ≠ 0
 polar
Resultant dipole moment = 0
 nonpolar

..

Non Polar molecule Polar molecule

 a polar molecule
 a polar molecule

Characteristic features of
polar and non-polar molecules

POLAR MOLECULES NON-POLAR MOLECULES

Bond dipole not cancel Bond dipole cancel each
each other other

Net dipole moment, Net dipole moment, Cl
Cl P Cl
N =0 Cl

≠0 Usually central atom has Cl
no lone pair
FF
Symmetrical molecule with
F Usually central atom has same terminal atoms
lone pair

Asymmetrical molecule
with different terminal
atoms

EXAMPLE:

1. ●●●●I●● ●●I●●●●

 EN = 0
 = 0
 I2 is a non-polar molecule

2. ●●●●C●●l ●●I●●●● ≠0

 EN ≠ 0
≠0

 ICl is a polar molecule

EXAMPLE:

●● ●●

3. ●O● C O●●

 CO2 shape: linear
 the two bond dipoles cancel each other
 resultant dipole moment,  = 0
 CO2 is a non-polar molecule

4. ●● ●● resultant  ≠ 0

O●● C ●S●

 OCS shape: linear

 the two bond dipole do not cancel each other

 resultant dipole moment,  ≠ 0

 OCS is a polar molecule

EXAMPLE:

●●●F●●●

5.

●● B ●●

●●●F● ●F●●●

 BF3 shape: trigonal planar
 the three bond dipoles cancel each other
 resultant dipole moment,  = 0
 BF3 is a non-polar molecule

EXAMPLE:

●●●F●●●

6. ●● B ●●

●●●F● B●●r ●● resultant  ≠ 0

 BF2Br shape: trigonal planar
 the three bond dipoles do not cancel each other
 resultant dipole moment,  ≠ 0
 BF2Br is a polar molecule

Summary on how to determine polarity of molecule

Draw lewis structure with correct geometry.

Are there polar bond present  ●● I ●● -
(show bond dipole)
●I ● ●I●

Yes No ●● C ●●

●O● ●O●

Are polar bonds Yes Are there lone
arranged so that pair on the
they can cancel? central atom

Yes No
No

Are the lone pair

Molecule is No on the central Yes Molecule is
POLAR atom are arrange NONPOLAR

so that they

cancel?

Exercise 2 EXERCISE

State whether the following molecules are polar or
non-polar.

a) H2O
b) NH3
c) CCl4
d) CHCl3

Answer:

polar molecules: H2O, NH3, CHCl3
non-polar molecule: CCl4