LEARNING OBJECTIVES

• Apply VSEPR to predict electronic

geometry and shapes of simple

molecules

• Distinguish between polar and nonpolar

bonds in molecules

• Predict polarity of simple molecules

from bond polarity and molecular shape

TOTAL NUMBER OF GROUPS

DICTATES ELECTRONIC GEOMETRY

Octet rule:

Two – linear

Three – trigonal planar

Four – tetrahedral

Additional possibilities (expand octet):

Five – trigonal bipyramidal

Six - octahedral

Electronic geometry considers

bonded atoms only.

Molecular geometry considers

unbonded pairs as well

MOLECULAR GEOMETRY

Molecular geometry is the three-dimensional

arrangement of a molecule’s atoms in space.

Linear

Bent

Trigonal-planar

Tetrahedral Trigonal-

pyramidal

Trigonal-bipyramidal Octahedral

VALENCE

SHELL

ELECTRON

PAIR

REPULSION

THEORY

VSEPR theory assumes that the shape of a

molecule is determined by the repulsion of

electron pairs.

VSEPR theory states

that repulsion between

the sets of valence-level

electrons surrounding an

atom causes these sets

to be oriented as far apart as possible.

Molecular Shape

VSEPR THEORY

• VSEPR (pronounced “vesper”) stands for Valence Shell Electron Pair Repulsion

• Based on Electron Dot (Lewis structures) • Theory predicts shapes of compounds based on electron pairs repelling (in bonds or by themselves)

• Electrons around central nucleus repel each other. So, structures have atoms maximally spread out

• Each shape has a name (you will have to memorize these)

• tetrahedral • trigonal pyramidal • bent • linear • trigonal planar

VSEPR AND MOLECULAR GEOMETRY

VSEPR AND MOLECULAR GEOMETRY

Molecule Lewis Structure Number of

electron pairs

CH4

NH3

SHAPE

Tetrahedral

Trigonal

Pyramidal

4

4

(3 shared

1 lone pair)

Molecule Lewis Structure Number of

electron pairs

H2O

CO2

SHAPE

Bent or V

4

(2 shared

2 lone pairs)

2

Linear

Molecule Lewis Structure Number of

electron pairs

BeCl2

BF3

SHAPE

2

3

Linear

Trigonal

Planar

Use VSEPR theory to predict the molecular

geometry of boron trichloride, BCl3.

First write the Lewis structure.

Boron is in Group 13 and has 3 valence

electrons.

Chlorine is in Group 17 so each chlorine atom

has 7 valence electrons.

The three B-Cl bonds stay farthest apart by pointing to the

corners of an equilateral triangle, giving 120o angles between

the bonds.

This would be trigonal-planar geometry.

methane, CH4

Bonds are all evenly spaced apart

Tetrahedral

109.5○

Less repulsion between the bonding pairs of electrons

.. ammonia

NH3

Trigonal Pyramidal

●●

surprise: the lone pairs occupy more space than the bonded atoms (with very few exceptions)

Two unbonded pairs of electrons make bond angles slightly less than tetrahedral due to

greater repulsion

.. water

H2O

bent

O H H

Bonded electrons can take maximum position apart – 180°

.. Carbon dioxide

CO2

linear

No lone pairs of electrons allows maximum bond angle in 1-D plane

.. Barium floride

BF3

Trigonal planar

The “z” plane – 3-D

22

The geometry of a

molecule refers to

the positions of

atoms only.

Water, H2O, has two unshared pairs, and its

molecular geometry takes the shape of a

“bent” or angular molecule.

Bent

• Unshared electron pairs repel other electron

pairs more strongly than bonding pairs do. • This is why the bond angles in ammonia and water are

somewhat less than the 109.5o bond angles of a perfectly

tetrahedral molecule.

The strongest intermolecular forces exist

between polar molecules.

•Polar molecules act as tiny dipoles. A dipole is created by equal but opposite charges that

are separated by a short distance. •The direction of a dipole is from the dipole’s positive pole to

its negative pole.

•A dipole is represented by an arrow with its head pointing

toward the negative pole and a crossed tail at the positive

pole.

•The dipole created by a hydrogen chloride molecule is

represented below:

MOLECULAR POLARITY AND DIPOLE-DIPOLE

FORCES

H Cl

The negative region in one

polar molecule attracts the

positive region in adjacent

molecules. So the molecules

all attract each other from

opposite sides.

The forces of attraction

between polar molecules

are known as dipole-

dipole forces.

Which atom attracts e- more?

H ― Cl δ+ δ-

electronegativities 2.1 3.0

C = O H H

2.5 3.5 2.1

2.1

O = C = O