Structures and Properties of Substances. Introducing Valence-Shell Electron- Pair Repulsion (VSEPR) Theory

(1)

Structures and Properties of Substances

Introducing Valence-Shell Electron-

Pair Repulsion (VSEPR) Theory

(2)

The VSEPR theory

In 1957, the chemist Ronald Gillespie and Ronald Nyholm, developed a model for predicting the shape of molecules. This model is usually abbreviated to VSEPR (pronounced “vesper”) theory:

V

^alence

S

^hell

E

^lectron

P

^air

R

^epulsion

The fundamental principle of the VSEPR theory is that the bonding pairs (BP) and lone pairs (LP) of electrons in the valence level of an atom repel one another.

Thus, the orbital for each electron pair is positioned as far from the other orbitals as possible in order to achieve the lowest possible unstable structure.

The effect of this positioning minimizes the forces of repulsion between electron pairs. A

(3)

The repulsion is greatest between lone pairs (LP-LP).

Bonding pairs (BP) are more localized between the atomic nuclei, so they spread out less than lone pairs. Therefore, the BP-BP repulsions are smaller than the LP-LP repulsions.

The repulsion between a bond pair and a lone-pair (BP-LP) is intermediate between the other two. In other words, in terms of decreasing repulsion:

LP-LP > LP-BP > BP-BP

The tetrahedral shape around a single-bonded carbon atom (e.g. in CH

4

), the planar shape around a carbon atom with two double bond (e.g. in CO

2

), and the bent shape around an oxygen atom in H

2

O result from repulsions

between lone pairs and/or bonding pairs of electrons.

The VSEPR theory

(4)

The repulsion is greatest between lone pairs (LP-LP).

Bonding pairs (BP) are more localized between the atomic nuclei, so they spread out less than lone pairs. Therefore, the BP-BP repulsions are smaller than the LP-LP repulsions.

The repulsion between a bond pair and a lone-pair (BP-LP) is intermediate between the other two. In other words, in terms of decreasing repulsion:

LP-LP > LP-BP > BP-BP

The tetrahedral shape around a single-bonded carbon atom (e.g. in CH

4

), the planar shape around a carbon atom with two double bond (e.g. in CO

2

), and the bent shape around an oxygen atom in H

2

O result from repulsions

between lone pairs and/or bonding pairs of electrons.

The VSEPR theory

(5)

Geometry of the molecules and the VSEPR theory

The figure below shows the five basic geometrical arrangements that result from the interactions of lone pairs and bonding pairs around a central atom.

These arrangements involve up to six electron groups. An electron group is usually one of the following:

• a single bond

• a double bond

• a triple bond

• a lone pair

When all the electron groups are BP, a molecule will have one of those five

geometrical arrangements. If one (or more) of the electron groups are LP,

variations in the geometric arrangements result.

(6)

Geometry of the molecules

Each of the molecules in the following pages below has four pairs of electrons around the central atom.

Observe the differences in the number of bonding and lone pairs in these molecules.

Methane, CH

4

, has 4 BP.

Ammonia, NH

3

, has 3 BP and 1 LP.

Water, H

2

O, has 2 BP and 2 LP.

These differences have an effect on the shapes and bond angles of the

molecules.

(7)

Geometry of the molecules

Methane

with four BP, has a tetrahedral molecular shape.

The angle between any two bonding pairs in the tetrahedral electron-group arrangement is 109.5°.

This angle corresponds to the most favorable arrangement of electron groups

to minimize the forces of repulsion among them.

(8)

Geometry of the molecules

Ammonia

When there are 1 LP and 3 BP around a central atom, there are two types of

repulsions:

LP-BP and BP-BP

Since LP-BP repulsions are greater than BP-BP repulsions, the bond angle between the bond pairs in NH

3

is reduced from 109.5 ̊ to 107.8 ̊.

When you draw the shape of a trigonal pyramidal molecule, without the lone pair, you can see that the three bonds form the shape of a pyramid with a

triangular base

(9)

Geometry of the molecules

Water

In a molecule of H

2

O, there are two BP and two LP.

The strong LP-LP repulsions, in addition to the LP-BP repulsions, cause the angle

between the bonding pairs to be reduced further to 104.5 ̊.

The result is the bent shape around an oxygen atom with 2 LP and two

single bonds

(10)

3 trigonal planar 2 BP, 1 LP AX2E SnCl2

4 tetrahedral 4 BP AX₄ CF₄

4 tetrahedral 3 BP, 1LP AX₃E PCl₃

4 tetrahedral 2 BP, 2LP AX₂E₂ H₂S

5 trigonal 5 BP AX₅ SbCl₅

bipyramidal

5 trigonal 4 BP, 1LP AX₄E TeCl₄

bipyramidal

X

X X

A X

tetrahedral

seesaw trigonal bipyramidal

trigonal pyramidal X A X

angular

• •

X X

A

X

• •

X A

X

• •

•

2 BP AX₂ BeF₂

2 3

linear

3 BP AX₃ BF₃

trigonal planar Geometric arrangement of electron groups Number of

electron groups Type of

electron pairs VSEPR notation Name of Molecular shape Example

X

X A X

trigonal planar X A X

linear

Table 4.2 Common Molecular Shapes and Their Electron Group Arrangements

X

X X

X X A

•• X

X X X

A

182 MHR • Unit 2 Structure and Properties

Common Molecular Shapes

(11)

Common Molecular Shapes

3 trigonal planar 2 BP, 1 LP AX₂E SnCl₂

bipyramidal

X

X X

A X

tetrahedral

angular

• •

X X

A

X

• •

X A

X

• •

2 BP AX₂ BeF₂

2 3

linear

3 BP AX₃ BF₃

X

X A X

linear

X

X X

X X A

•

• X

X X X

A

3 trigonal planar 2 BP, 1 LP AX₂E SnCl₂

bipyramidal

X

X X

A X

tetrahedral

angular

• •

X X

A

X

• •

X A

X

• •

•

2 BP AX₂ BeF₂

2 3

linear

3 BP AX3 BF3

X

X A X

linear

X

X X

X X A

•

• X

X X X

A

Chapter 4 Structures and Properties of Substances • MHR 183

Predicting Molecular Shape

You can use the steps below to help you predict the shape of a molecule (or polyatomic ion) that has one central atom. Refer to these steps as you work through the Sample Problems and the Practice Problems that follow.

1. Draw a preliminary Lewis structure of the molecule based on the formula given.

2. Determine the total number of electron groups around the central atom (bonding pairs, lone pairs and, where applicable, account for the charge on the ion). Remember that a double bond or a triple bond is counted as one electron group.

3. Determine which one of the five geometric arrangements will accommodate this total number of electron groups.

4. Determine the molecular shape from the positions occupied by the bonding pairs and lone pairs.

3 BP, 2LP AX₃E₂ BrF₃

5

trigonal bipyramidal

2 BP, 3LP AX₂E₃ XeF₂

trigonal bipyramidal

linear

6 octahedral 6 BP AX₆ SF₆

octahedral

6 octahedral 5 BP, 1LP AX5E BrF5

square pyramidal

6 octahedral 4 BP, 2LP AX₄E₂ XeF₄

square planar T-shaped

A

••

• •

X X X

A

••

• •

X X

X

X X

X

A

X X X

X

• •

A

X X X

X

• •

•

A

(12)

Predicting Molecular Shape

It is possible to use the steps below to predict the shape of a molecule (or polyatomic ion) that has one central atom.

1.Draw a preliminary Lewis structure of the molecule based on the formula given.

2.Determine the total number of electron groups around the central atom (bonding pairs, lone pairs and, where applicable, account for the charge on the ion). Remember that a double bond or a triple bond is counted as one electron group.

3.Determine which one of the five geometric arrangements will accommodate this total number of electron groups.

4.Determine the molecular shape from the positions occupied by the

bonding pairs and lone pairs.

(13)

Sample Problem

Problem

Determine the molecular shape of the hydronium ion, H₃O⁺.

Plan Your Strategy

Follow the four-step procedure that helps to predict molecular shape.

Use Table 4.2 for names of the electron-group arrangements and molecular shapes.

Act on Your Strategy

Step 1 A possible Lewis structure for H₃O⁺ is:

Step 2 The Lewis structure shows 3 BPs and 1 LP. That is, there are a total of four electron groups around the central O atom.

Step 3 The geometric arrangement of the electron groups is tetrahedral.

Step 4 For 3 BPs and 1 LP, the molecular shape is trigonal pyramidal.

Check Your Solution

This molecular shape corresponds to the VSEPR notation for this ion, AX₃E.

+

O• • H H

H

Sample Problem

Predicting Molecular Shape for a Simpler Compound

Problem

Determine the shape of SiF₆²⁻ using VSEPR theory.

Plan Your Strategy

Follow the four-step procedure that helps to predict molecular shape apply. Use Table 4.2 for names of the electron group arrangements and molecular shapes.

Act on Your Strategy

Step 1 Draw a preliminary Lewis structure for SiF₆²⁻.

This polyatomic ion has six bonds around the central Si atom, an obvious exception to the octet rule, so the central atom needs an expanded valence shell.

Total number of valence electrons

= 1 Si atom × 4 e⁻/Si atom + 6 F atom × 7 e⁻/F atom + 2 e⁻ (ionic charge)

= 48 e⁻

F F

Si

Sample Problem

Predicting Molecular Shape for a Complex Compound

Determine the molecular shape of the hydronium ion, H3O⁺ 1.Plan Your Strategy

Follow the four-step procedure that helps to predict molecular shape. Use the “Common molecular shapes” table on the previous pages for names of the electron-group

arrangements and molecular shapes.

2.Act on Your Strategy

Step 1: A possible Lewis structure for H3O⁺ is:

Step 2: The Lewis structure shows 3 BPs and 1 LP. That is, there are a total of four electron groups around the central O atom.

Step 3: The geometric arrangement of the electron groups is tetrahedral.

Step 4: For 3 BP and 1 LP, the molecular shape is trigonal pyramidal.

This molecular shape corresponds to the VSEPR notation for this ion, AX3E .

(14)

Practice Problem

•

Use VSEPR theory to predict the molecular shape for each of the following:

(a) HCN (b) SO2 (c) SO3 (d) SO42-

•

Use VSEPR theory to predict the molecular shape for each of the following:

(a) CH2F2 (b) NH4+ (c) BF4-

•

Use VSEPR theory to predict the molecular shapes of NO2+ and NO2-.