VSEPR THEORYVSEPR THEORY (Valence Shell (Valence Shell Electron Pair Electron Pair

Repulsion Theory)Repulsion Theory)Adapted by Mr. M. McIsaacAdapted by Mr. M. McIsaac

Carleton North High School, Carleton North High School, Bristol, NBBristol, NB

From Mr. James MontgomeryFrom Mr. James MontgomerySCH4U Grade 12 ChemistrySCH4U Grade 12 Chemistry

Sir John A. MacDonald Secondary SchoolSir John A. MacDonald Secondary SchoolWaterloo, ONWaterloo, ON

sjam.wrdsb.on.ca/files/Lesson14sjam.wrdsb.on.ca/files/Lesson14vseprvsepr.ppt .ppt

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What Is The VSEPR Theory?What Is The VSEPR Theory? VSEPR Theory is used to predict the VSEPR Theory is used to predict the

shapes of molecules.shapes of molecules. Think of bonded pairs (shared) or lone Think of bonded pairs (shared) or lone

pairs (nonbonded, unshared) of epairs (nonbonded, unshared) of e--’s as ’s as negatively charged clouds that repel negatively charged clouds that repel each other.each other.

To achieve the most stable condition the To achieve the most stable condition the clouds must be as far apart as possible clouds must be as far apart as possible in 3-D, thereby decreasing repulsion.in 3-D, thereby decreasing repulsion.

The amount of repulsion can be ordered:The amount of repulsion can be ordered:LP-LP > LP-BP > BP-BPLP-LP > LP-BP > BP-BP

In order to determine the shape, the In order to determine the shape, the Lewis diagram must be drawn first.Lewis diagram must be drawn first.

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2 Bond Pairs/Electron Groups2 Bond Pairs/Electron Groups Molecules that only have 2 bonding pairs Molecules that only have 2 bonding pairs

on the central atom will have a LINEAR on the central atom will have a LINEAR SHAPE with a bond angle of 180SHAPE with a bond angle of 180°°

e.g. BeFe.g. BeF22, CO, CO22, CS, CS22

General Formula: AXGeneral Formula: AX22

Central atom A from group 2; 2 BP 0 LPCentral atom A from group 2; 2 BP 0 LP

Be FF180°

120°

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3 Bond Pairs/Electron 3 Bond Pairs/Electron GroupsGroups

Molecules that have 3 bonding pairs on Molecules that have 3 bonding pairs on the central atom will have a TRIGONAL the central atom will have a TRIGONAL PLANAR SHAPE with bond angles of 120PLANAR SHAPE with bond angles of 120°.°.

e.g. BFe.g. BF33, BH, BH33 General Formula: AXGeneral Formula: AX33

Central atom A from group 13; 3 BP 0 Central atom A from group 13; 3 BP 0 LPLP

BFF

F

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4 Bonding Pairs/Electron 4 Bonding Pairs/Electron GroupsGroups

If the central atom is placed at the center If the central atom is placed at the center of a sphere, than each of the four pairs of of a sphere, than each of the four pairs of electrons will occupy a position to be as far electrons will occupy a position to be as far apart as possible.apart as possible.

This will result in the electron pairs being This will result in the electron pairs being at the corners of a regular tetrahedron, at the corners of a regular tetrahedron, therefore these molecules are said to have therefore these molecules are said to have a a TETRAHEDRAL SHAPE.TETRAHEDRAL SHAPE.

The angle between each bond will be 109.5The angle between each bond will be 109.5°° e.g. CCle.g. CCl44, CH, CH44, SiH, SiH44

General Formula: AXGeneral Formula: AX44

Central atom A from group 14; 4 BP 0 Central atom A from group 14; 4 BP 0 LPLP

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Example CClExample CCl44

109.5°

Cl

C

Cl

Cl

Cl

Cl

CCl Cl

Cl

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3 Bonding Pairs & 1 Non-3 Bonding Pairs & 1 Non-bonding Pairbonding Pair

Four pairs of electrons will always arrange Four pairs of electrons will always arrange themselves tetrahedrally around the central themselves tetrahedrally around the central atom.atom.

The shape of the molecule is determined by The shape of the molecule is determined by the arrangement of the atoms not the the arrangement of the atoms not the electrons.electrons.

As a result such molecules will have a As a result such molecules will have a TRIGONAL PYRAMIDALTRIGONAL PYRAMIDAL shape. shape.

Due to the repulsion, a non-bonding electron Due to the repulsion, a non-bonding electron pair requires more space than a bonding pair requires more space than a bonding pair, the angles in these molecules are pair, the angles in these molecules are 107107° ° not 109.5° as in the tetrahedral molecules.not 109.5° as in the tetrahedral molecules.

e.g. NHe.g. NH33, PCl, PCl33 General Formula: AXGeneral Formula: AX33EE Central atom A from group 15; 3 BP 1 LPCentral atom A from group 15; 3 BP 1 LP

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Example NHExample NH33

NH H

H

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2 Bonding Pairs & 2 Non-2 Bonding Pairs & 2 Non-bonding Pairsbonding Pairs

The four pairs of electrons will be arranged The four pairs of electrons will be arranged tetrahedrally but since only 2 pairs are tetrahedrally but since only 2 pairs are bonding electrons, the surrounding atoms bonding electrons, the surrounding atoms are at 2 corners of the tetrahedron.are at 2 corners of the tetrahedron.

As a result these molecules will have a As a result these molecules will have a V-V-SHAPE or BENT.SHAPE or BENT.

The repulsion between the non-bonding The repulsion between the non-bonding pairs will result in a bond angle of 104.5pairs will result in a bond angle of 104.5°.°.

For each pair of non-bonding electrons, the For each pair of non-bonding electrons, the bond angle decreases by 2.5°bond angle decreases by 2.5°

e.g. He.g. H22O, HO, H22S, OClS, OCl22

General Formula: AXGeneral Formula: AX22EE22

Central atom A from group 16; 2 BP 2 Central atom A from group 16; 2 BP 2 LPLP

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Example HExample H22OO

HO

H

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Things to RememberThings to Remember In order to predict the shape of a In order to predict the shape of a

molecule you must draw the Lewis Dot molecule you must draw the Lewis Dot Diagram for the molecule, determine Diagram for the molecule, determine the number of bonding and non-the number of bonding and non-bonding electron pairs and compare bonding electron pairs and compare this with the chart you have been given this with the chart you have been given (the shapes must be memorised).(the shapes must be memorised).

When determining the shape of a When determining the shape of a molecule with multiple bonds, treat the molecule with multiple bonds, treat the multiple bonds as if they were single multiple bonds as if they were single bonds (i.e. one bonding pair)bonds (i.e. one bonding pair)

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5 Molecular ShapesFrom

Single Bonds