chapter 13 molecular shapes vsepr theory

Chapter 13

Molecular Shapes

VSEPR Theory

Molecular Shape

• Molecular shape or molecular geometry

is the three-dimensional arrangement of

the atoms in a molecule.

Molecular Shape

• Molecular shape determines several

properties of a substance including:

– reactivity

– polarity

– phase of matter

– color

– magnetism

– biological activity

Olfaction – sense of smell

• Lock-and-Key Theory: humans can

smell various odors because each three-

dimensional odor molecule fits into only

one type of receptor.

Gustation – the sense of taste

• Taste receptors are

located on the tongue

and are sensitive to four

major tastes: salty,

sweet, sour and bitter.

• Taste receptors respond

differentially to the

varying shapes of the

food molecules that we

eat.

H2

H H

2(1) = 2e-

Any 2 atoms

linear

What about molecules consisting of

more than two atoms

For this we must use the VSEPR Theory

VSEPR Theory

• Valence Shell Electron Pair Repulsion Theory

• Predicts the molecular shape of a bonded

molecule containing a central atom(s).

• Electrons around the central atom arrange

themselves as far apart from each other as

possible

• So only electrons (lone pairs or bonds)

connected to the central atom are important.

Five Molecular Shapes

See the table in your notes

Steps in Determining Molecular Shape

1. Draw the Lewis Structure for the

Molecule.

2. Count the number of atoms attached to

the central atom.

3. Count the number of lone pairs attached

to the central atom.

4. Use your counts on steps 2 and 3 to

determine the shape of the molecule.

H2S

• Dihydrogen monosulfide is commonly known as

sewer gas. This colorless, toxic and flammable

gas is responsible for the foul odor of rotten eggs.

H HS••

••

2(1)+6 = 8e-

2/2

angular

H2S

• Dihydrogen monosulfide is commonly known as

sewer gas. This colorless, toxic and flammable

gas is responsible for the foul odor of rotten eggs.

H HS

Methane (CH4)4 + 4(1) = 8e-

Methane (CH4)4 + 4(1) = 8e-

4 atoms attached to the

central atom.

0 lone pairs on the

central atom.

4/0

Tetrahedral shape

HCN

H NC••

••

1+4+5 = 10e-

••

••

••

HCN

H NC••

1+4+5 = 10e-

••

••

HCN

H NC

1+4+5 = 10e-

••

2/0

linear

H2S

H H

S

2/2

angular

SO2

O OS••

••

6+2(6) = 18e-

••

••

••••

••••

SO2

O OS••

6+2(6) = 18e-

••

••••

••••

2/1

angular

CH4

4/0

tetrahedral

NO3-

O ON••

••

5+3(6)+1 = 24e-

••

••

••••

••••

O ••

••

NO3-

O ON

5+3(6)+1 = 24e-

••

••

••••

••••

O ••

•• 3/0

trigonal

planer

[ ]-

Draw the Lewis structure

for an acetate ion?You learned acetate as C2H3O2

-.

However writing it as CH3CO2- will give you a

better understanding of its Lewis structure.

Draw the Lewis structure

for an acetate ion?

What’s the shape of acetate?

4/0

tetrahedral

3/0

trigonal

planer

chapter 13 molecular shapes vsepr theory

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