Molecular Structure: the VSEPR Model

1. Molecular Structure: the VSEPR Model • Valence Shell electron-pair repulsion VSEPR model • Useful to predict geometry of a molecule • Postulate: The structure around an atom is determined by minimizing electron pair repulsions (both bonding and nonbonding)

2.     BeCl2 Angle = 180o   Cl Cl Be   2 electron pairs: Best arrangement is Linear

3.          F Angle = 120o B   F F   3 electron pairs: Best arrangement is Trigonal Planar with 120o angles

4. H 4+4=8 electrons C H H Angle = 109.5o H 4 effective electron pairs: Best arrangement is tetrahedral Angle = 109.5o Electron arrangement:tetrahedral Geometry: tetrahedral

5.  N H H H The tetrahedral arrangement of electron pairs around the nitrogen atom in the ammonia molecule.Molecular geometry is trigonal pyramidal, not tetrahedral. 5+3=8 electrons

6. The tetrahedral arrangement of the four electron pairs around oxygen in the water molecule. The V-shaped molecular structure of the water molecule. 6+2=8 electrons   O H  H

7. The bond angles in the CH4, NH3, and H2O molecules.

8. In a bonding pair of electrons, the electrons are shared by two nuclei. In a lone pair, both electrons must be close to a single nucleus and tend to take up more of the space around that atom

10. PCl5 Cl Cl Angle = 90o P Cl Angle = 120o Cl Cl 5 electron pairs: Best arrangement is trigonal bipyramid

11. Molecular structure of PCl6- (and SF6) Octahedral Geometry

12. Molecule Geometry: Central Atoms without Lone Pairs

13. .. .. .. .. _ I I I .. .. .. .. .. Central Atom with Lone Pairs Three possible arrangements of the electron pairs in the I3- ion. axial equatorial Electron arrangement: Trigonal bipyramid Geometry: Linear To minimize electron pair repulsion place the lone pairs in equatorial, not axial positions

14. electron-pair arrangement and geometry of XeF4 F F 6 effective electron pairs   Xe   F F Octahedral electron arrangement Geometry: Square Planar

15. Summary of 5 e-pairs 5 effective pairs: 1 lone pair- See-Saw 5 effective pairs: 2 lone pairs T-Shaped 5 effective pairs: 3 lone pairs Linear            

16. Summary of 6 e-pairs 6 effective pairs: 1 lone pair- Square Pyramidal 6 effective pairs: 2 lone pairs Square Planar

17. Central Atoms with Lone Pairs

18.     O     N     O O     Multiple Bonds • A multiple bond counts as one effective electron pair NO3- _ 3 effective electron pairs: Best arrangement is trigonal planar with 120o

19.     • Note: angles of 120o or more do not result in appreciable bond distortion: e.g. SO2 (18 valence electrons)   Compare to CO2 Linear, nonpolar SO2, bent, polar S     O O   3 effective electron pairs: Best electron arrangement is trigonal planar with 120o Geometry is BENT with 1200 (also known as V-shaped or Angular)

20. More than one central atomMethanol: CH3OH H  H H C O  H

21. VSEPR Does not always workIn the end the experiment determines the geometry Example is PH3 phosphine:  P H H Actual Angle is 94o Not as predicted by VSEPR (like NH3 ~ 107o) H