VSEPR THEORY (Valence Shell Electron Pair Repulsion Theory)

Adapted by Mr. M. McIsaac Carleton North High School, Bristol, NB From Mr. James Montgomery SCH4U Grade 12 Chemistry Sir John A. MacDonald Secondary School Waterloo, ON

sjam.wrdsb.on.ca/files/Lesson14vsepr.ppt

What Is The VSEPR Theory?

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VSEPR Theory is used to predict the shapes of molecules.

Think of bonded pairs (shared) or lone pairs (nonbonded, unshared) of e ’s as negatively charged clouds that repel each other.

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

The amount of repulsion can be ordered: LP-LP > LP-BP > BP-BP In order to determine the shape, the Lewis diagram must be drawn first.

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2 Bond Pairs/Electron Groups

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Molecules that only have 2 bonding pairs on the central atom will have a LINEAR SHAPE with a bond angle of 180 ° e.g. BeF 2 , CO 2 , CS 2 General Formula: AX 2 Central atom A from group 2; 2 BP 0 LP

F 180 ° Be F 3

3 Bond Pairs/Electron Groups

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

e.g. BF 3 , BH 3 General Formula: AX 3 Central atom A from group 13; 3 BP 0 LP

F F B 120 ° F 4

4 Bonding Pairs/Electron Groups

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

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

The angle between each bond will be 109.5

° e.g. CCl 4 , CH 4 , SiH 4 General Formula: AX 4 Central atom A from group 14; 4 BP 0 LP

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Example CCl 4

Cl Cl C Cl Cl

109.5

° Cl Cl C Cl Cl 6

3 Bonding Pairs & 1 Non-bonding Pair

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

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

As a result such molecules will have a PYRAMIDAL shape.

TRIGONAL Due to the repulsion, a non-bonding electron pair requires more space than a bonding pair, the angles in these molecules are 107 ° not 109.5

° as in the tetrahedral molecules.

e.g. NH 3 , PCl 3 General Formula: AX 3 E Central atom A from group 15; 3 BP 1 LP

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Example NH 3

H N H H 8

2 Bonding Pairs & 2 Non-bonding Pairs

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

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

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

° .

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

° e.g. H 2 O, H 2 S, OCl 2 General Formula: AX 2 E 2 Central atom A from group 16; 2 BP 2 LP

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Example H 2 O

H O H 10

Things to Remember

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

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

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5 Molecular Shapes From Single Bonds

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