structures and properties of substances · 2016. 2. 15. · with four bp, has a tetrahedral...

Structures and Properties of Substances

Introducing Valence-Shell Electron-Pair Repulsion (VSEPR) Theory

The VSEPR theoryIn 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:

Valence

Shell

Electron

Pair

Repulsion

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

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 CH4), the planar shape around a carbon atom with two double bond (e.g. in CO2), and the bent shape around an oxygen atom in H2O result from repulsions between lone pairs and/or bonding pairs of electrons.

The VSEPR theory

Geometry of the molecules and the VSEPR theoryThe 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.

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, CH4, has 4 BP. Ammonia, NH3, has 3 BP and 1 LP. Water, H2O, has 2 BP and 2 LP.

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


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 NH3 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


Water In a molecule of H2O, 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

3 2 BP, 1 LP AX2E SnCl2trigonal planar

4 4 BP AX4 CF4tetrahedral

4 3 BP, 1LP AX3E PCl3tetrahedral

4 2 BP, 2LP AX2E2 H2Stetrahedral

5 5 BP AX5 SbCl5trigonal bipyramidal

5 4 BP, 1LP AX4E TeCl4trigonal bipyramidal

X

XX

XA

tetrahedral

seesaw

trigonal bipyramidal

trigonal pyramidal

X

AX

angular

angular

••

X X

A

X

• •

XA

X

• ••

•

2 BP AX2 BeF22

3

linear

3 BP AX3 BF3trigonal planar

Geometric arrangementof electron groups

Number of electron groups

Type of electron pairs VSEPR notation ExampleName of Molecular shape

X

X

AX

trigonal planar

XAXlinear

Table 4.2 Common Molecular Shapes and Their Electron Group Arrangements

X

X

X

X

X A

•• X

XX

X

A

182 MHR • Unit 2 Structure and Properties

Common Molecular Shapes

Common Molecular Shapes







X

XX

XA

tetrahedral

seesaw


trigonal pyramidal

X

AX

angular

angular

••

X X

A

X

• •

XA

X

• ••

•

2 BP AX2 BeF22

3

linear





X

X

AX

trigonal planar

XAXlinear


X

X

X

X

X A

•• X

XX

X

A








X

XX

XA

tetrahedral

seesaw


trigonal pyramidal

X

AX

angular

angular

••

X X

A

X

• •

XA

X

• •

••

2 BP AX2 BeF22

3

linear





X

X

AX

trigonal planar

XAXlinear


X

X

X

X

X A

•• X

XX

X

A


Chapter 4 Structures and Properties of Substances • MHR 183

Predicting Molecular ShapeYou 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 youwork through the Sample Problems and the Practice Problems that follow.

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

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

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

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

3 BP, 2LP AX3E2 BrF35

5


2 BP, 3LP AX2E3 XeF2trigonal bipyramidal

linear

6 6 BP AX6 SF6octahedral

octahedral

6 5 BP, 1LP AX5E BrF5octahedral

square pyramidal

6 4 BP, 2LP AX4E2 XeF4octahedral

square planar

T-shaped

A

••

••

X

X

X

A

••

••

••

X

X

X

X

XX

X

X

A

X

XX

X

X

• •

A

X

XX

X

• •

••

A

Predicting Molecular ShapeIt 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.

Sample Problem


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

Plan Your StrategyFollow 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 StrategyStep 1 A possible Lewis structure for H3O+ is:

Step 2 The Lewis structure shows 3 BPs and 1 LP. That is, there are atotal 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 ProblemPredicting Molecular Shape for a Simpler Compound

ProblemDetermine the shape of SiF6

2− using VSEPR theory.

Plan Your StrategyFollow the four-step procedure that helps to predict molecular shapeapply. Use Table 4.2 for names of the electron group arrangements andmolecular shapes.

Act on Your StrategyStep 1 Draw a preliminary Lewis structure for SiF6

2− .

This polyatomic ion has six bonds around the central Si atom, an obvious exception to the octet rule, so the central atom needsan 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

FF

FF

Si

Sample ProblemPredicting 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 .

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-.

structures and properties of substances · 2016. 2. 15. · with four bp, has a tetrahedral...

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