Valence Shell Electron Pair Repulsion Theory

M.SARAVANAKUMARM.TECH –I YEAR

Centre for Nanoscience and Technology, Pondicherry University

11/9/2012

INTRODUCTION

• VSEPR Theory was suggested by Sidgwick and Powel[1940]

• It was developed by Gilllespe and Nyholm in 1957.

• Based on that in a polyatomic molecule the direction bonds around the central atom depends on the total number of Bonding &Non-bonding electron pairs in its valance shell.

VSEPR Theory

• The shape of the molecule is determined by repulsions between all of the electron present in the valance shell.

• Electron pairs in the valence shell of the central atom repel each other and align themselves to minimize this repulsion.

• Lone pair electrons takes up more space round the central atom than a bondpair.

• Lone pair attracted to one nucleus, but bond pair is shared by two nuclei.

• The minimum repulsions to the state minimum energy and maximum stability of the molecule.

Repulsion strengths

• Lone pair -Lone pair Lone pair -Bond pair Bond pair-Bond pair

Repulsion strengths

Triple bond >double bond >single bond

• Presence of lone pairs on the central atom causes slight distortion of the bond angles from the ideal shape.

• The magnitude of repulsions between bonding pairs of electrons depends on the electronegativity difference between the central atom &the other atoms .

Steps in the prediction of geometry

• Determine the central atom.

• Draw the electron dot structure and bar diagram

• Find arrangement of electron pairs.

• Find arrangement of bonding pairs.

• Determine the geometry based on of bonding pairs.

SHAPES OF MOLECULES CONTAINING

BONDED PAIRS OF ELECTRONS ONLY

Central atom with Two Electron Pairs

There are two electron pairs in the valance shell of Beryllium. [1s2 2s2 ]

Molecular geometry-Linear arrangement

H HBe

Central atom with Three Electron PairsTrigonal Planar

Three electron pairs in the valance shell of Boron. [1s2 2s2 2p1 ]

Molecular geometry- Trigonal Planar arrangement

BF

F

F

Central atom with Four Electron PairsTetrahedral

Four electron pairs in the valance shell of Carbon. [1s2 2s2 2p2 ]

Molecular geometry- Tetrahedral Bond angle -109.5⁰

Central atom with Five Electron Pairs Five electrons in the valance shell of

Phosphorus. [1s2 2s2 2p6 3s2 3p3 ] Molecular geometry- Trigonalbipyramid

Bond angle -120⁰ &90⁰

Central atom with Six Electron Pairs

Six electrons in the valance shell of Sulphur. [1s2 2s2 2p6 3s2 3p4 ]

Molecular geometry- OctahetralBond angle-90⁰

SHAPES OF MOLECULES CONTAINING BONDED PAIRS AS WELL AS LONE PAIRS

OF ELECTRONS

Central atom with Four Electron PairsTetrahedral

Five electrons in the valance shell of Nitrogen. [1s2 2s2 2p3]

Molecular geometry- Tetrahedral Electron pairarrangement - Trigonal Pyramidal

Presence of lonepair causes slight distortion from 109⁰28 to 107⁰48′ ′

Central atom with Four Electron PairsTetrahedral

Six electrons in the valance shell of Oxygen atom. [1s2 2s2 2p4 ]

Molecular geometry- V-Shaped or Bent shape Electron pairarrangement- Tetrahetral Repulsions between Lonepair- Lonepair ,Lonepair -Bondpair is possible. It causes slight distortion from 109⁰28 to 104⁰27′ ′

Central atom with Five Electron Pairs

Five electrons in the valance shell of Sulphur. [1s2 2s2 2p6 3s2 3p4 ]

Molecular geometry- seesaw[or distorted octahedron]

Electron pair arrangement- Trigonalbipyramidal

Lonepair –bondpair repulsion

Central atom with Six Electron Pairs Seven electrons in the valance shell of

xenon : 1s2 2s2 2p6 3s2 3p6 3d104s2 4p6 4d10 5s2 5p6

Molecular geometry- square planar Electron pairarrangement- octahedral

Lonepair –bondpair repulsion

EFFCT OF ELECTRONEGATIVITY Five electrons in the valance shell of

Nitrogen. [1s2 2s2 2p3] Molecular geometry- Tetrahedral [e-pairs arrangement]

orTrigonal Pyramidal[VSEPR]Presence of lonepair causes slight distortion from 109⁰28 to ′

107⁰48 to102⁰30′ ′Repulsion between bonpair-bondpair is less in NF3 than inNH3

102⁰30′

Limitations of VSEPR

• It fails to predict the shapes of isoelectronic species[CH4&NH4

+] and transition metal compounds.

• This model does not take relative sizes of substituents .

• Unable to explain atomic orbitals overlap.

Seven electrons in the valance shell of xenon : 1s2 2s2 2p6 3s2 3p6 3d104s2 4p6 4d10 5s2 5p6

Molecular geometry- Pentagonalbipyramid[VSEPR], DistortedOctahedral[actual]

Lonepair –bondpair repulsion

REFERENCES

• Concise Inorganic chemistry-J.D.LEE[Fifth edition]-Black well publishing.

• Chemistry-Raymond chang. • Pictures from Wikipedia- free encyclopedia-

www. Wikipedia.org.• Pictures from Google Images.

Thanks to my Teachers & friends

THANK YOU