Vsepr theory

1. Valence Shell Electron Pair Repulsion Theory M.SARAVANAKUMAR M.TECH –I YEAR Centre for Nanoscience and Technology, Pondicherry University 11/9/2012

2. 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.

3. 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.

4. Repulsion strengths Lone pair -Lone pair  Lone pair -Bond pair  Bond pair-Bond pair

5. Repulsion strengths Triple bond > double bond > single bond

6. 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 .

7. 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.

8. SHAPES OF MOLECULES CONTAINING BONDED PAIRS OF ELECTRONS ONLY

9. Central atom with Two Electron Pairs There are two electron pairs in the valance shell of Beryllium . [1s 2 2s 2 ] Molecular geometry- Linear arrangement H H Be

10. Central atom with Three Electron Pairs Trigonal Planar Three electron pairs in the valance shell of Boron . [1s 2 2s 2 2p 1 ] Molecular geometry- Trigonal Planar arrangement B F F F

11. Central atom with Four Electron Pairs Tetrahedral Four electron pairs in the valance shell of Carbon . [1s 2 2s 2 2p 2 ] Molecular geometry- Tetrahedral Bond angle -109.5⁰

12. Central atom with Five Electron Pairs Five electrons in the valance shell of Phosphorus . [1s 2 2s 2 2p 6 3s 2 3p 3 ] Molecular geometry- Trigonalbipyramid Bond angle -120⁰ &90⁰

13. Central atom with Six Electron Pairs Six electrons in the valance shell of Sulphur . [1s 2 2s 2 2p 6 3s 2 3p 4 ] Molecular geometry- Octahetral Bond angle-90⁰

14. SHAPES OF MOLECULES CONTAINING BONDED PAIRS AS WELL AS LONE PAIRS OF ELECTRONS

15. Central atom with Four Electron Pairs Tetrahedral Five electrons in the valance shell of Nitrogen . [1s 2 2s 2 2p 3 ] Molecular geometry- Tetrahedral Electron pairarrangement - Trigonal Pyramidal Presence of lonepair causes slight distortion from 109⁰28′ to 107⁰48′

16. Central atom with Four Electron Pairs Tetrahedral Six electrons in the valance shell of Oxygen atom. [1s 2 2s 2 2p 4 ] 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′

17. Central atom with Five Electron Pairs Five electrons in the valance shell of Sulphur . [1s 2 2s 2 2p 6 3s 2 3p 4 ] Molecular geometry- seesaw[or distorted octahedron] Electron pair arrangement- Trigonalbipyramidal Lonepair – bondpair repulsion

18. Central atom with Six Electron Pairs Seven electrons in the valance shell of xenon  : 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2 4p 6 4d 10 5s 2 5p 6 Molecular geometry- square planar Electron pairarrangement - octahedral Lonepair – bondpair repulsion

19. EFFCT OF ELECTRONEGATIVITY Five electrons in the valance shell of Nitrogen . [1s 2 2s 2 2p 3 ] Molecular geometry- Tetrahedral [ e-pairs arrangement ] or Trigonal Pyramidal [VSEPR] Presence of lonepair causes slight distortion from 109⁰28′ to 107⁰48′to102⁰30′ Repulsion between bonpair-bondpair is less in NF 3 than inNH 3 102⁰30′

20. Limitations of VSEPR  It fails to predict the shapes of isoelectronic species [CH 4 &NH 4 + ] and transition metal compounds.  This model does not take relative sizes of substituents . Unable to explain atomic orbitals overlap.

21. Seven electrons in the valance shell of xenon  : 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2 4p 6 4d 10 5s 2 5p 6 Molecular geometry- Pentagonalbipyramid [VSEPR], DistortedOctahedral [actual] Lonepair – bondpair repulsion

22. 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

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