VSEPR Theory and Molecular Geometry

1. Mr. Chapman Chemistry 20 VSEPR Theory and Molecular Geometry

2. Back to the Basics • It seems like we have known since the beginning of our education that opposites attractand that particles of the same charge repel each other. • This simple fact is the basis for why molecules form in the shapes that they do, and it is called VSEPR theory. VSEPR stands for Valence Shell Electron Pair Repulsion Theory.

3. Molecular Shape • Molecules have definite shapes and the shape of a molecule controls some of its chemical and physical properties. • What is the difference between a chemical and physical property? • There are some important definitions we must understand in order to comprehend molecular geometry.

4. Definitions • Bond lengths – the distances between the nuclei of bonded atoms • Bond angles –is the angle formed between three atoms across at least two bonds. • Bond lengths and bond angles are determined by the pairs of electrons that are in the valence shell of the central atom.

5. Molecules Can Take on a Number of Different Shapes

6. Valence-Shell Electron-Pair Repulsion (VSEPR) Theory • Electron pairs repel each other, whether they are in chemical bonds (bond pairs) or unshared (lone pairs). • Electron pairs assume orientations about an atom to minimize repulsions. • In other words, electrons in the valence shell want to get as far away from each other as possible.

7. Important Point • There is an important difference between electron pair arrangement and molecular geometry, which we are going to see in the next few slides. • Electron pair arrangement determines the molecular geometry of the molecule.

8. # of atoms bonded tocentral atom # lone pairs on central atom Arrangement ofelectron pairs Molecular Geometry Class linear linear B B Valence shell electron pair repulsion (VSEPR) model: Predict the geometry of the molecule from the electrostatic repulsions between the electron (bonding and nonbonding) pairs. AB2 2 0 10.1

9. 0 lone pairs on central atom Cl Be Cl 2 atoms bonded to central atom 10.1

10. # of atoms bonded tocentral atom # lone pairs on central atom trigonal planar trigonal planar Arrangement ofelectron pairs Molecular Geometry Class VSEPR AB2 2 0 linear linear AB3 3 0 10.1

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12. # of atoms bonded tocentral atom # lone pairs on central atom trigonal planar trigonal planar AB3 3 0 Arrangement ofelectron pairs Molecular Geometry Class tetrahedral tetrahedral VSEPR AB2 2 0 linear linear AB4 4 0 10.1

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14. # of atoms bonded tocentral atom # lone pairs on central atom trigonal planar trigonal planar AB3 3 0 Arrangement ofelectron pairs Molecular Geometry Class trigonal bipyramidal trigonal bipyramidal VSEPR AB2 2 0 linear linear tetrahedral tetrahedral AB4 4 0 AB5 5 0 10.1

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16. # of atoms bonded tocentral atom # lone pairs on central atom trigonal planar trigonal planar AB3 3 0 Arrangement ofelectron pairs Molecular Geometry Class trigonal bipyramidal trigonal bipyramidal AB5 5 0 octahedral octahedral VSEPR AB2 2 0 linear linear tetrahedral tetrahedral AB4 4 0 AB6 6 0 10.1

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18. # of atoms bonded tocentral atom # lone pairs on central atom trigonal planar Arrangement ofelectron pairs Molecular Geometry bent Class VSEPR trigonal planar trigonal planar AB3 3 0 AB2E 2 1 10.1

19. # of atoms bonded tocentral atom # lone pairs on central atom trigonal pyramidal tetrahedral Arrangement ofelectron pairs Molecular Geometry Class VSEPR tetrahedral tetrahedral AB4 4 0 AB3E 3 1 10.1

20. # of atoms bonded tocentral atom # lone pairs on central atom trigonal pyramidal Arrangement ofelectron pairs Molecular Geometry AB3E 3 1 tetrahedral Class bent tetrahedral O H H VSEPR tetrahedral tetrahedral AB4 4 0 AB2E2 2 2 10.1

21. # of atoms bonded tocentral atom # lone pairs on central atom trigonal bipyramidal distorted tetrahedron Arrangement ofelectron pairs Molecular Geometry Class VSEPR trigonal bipyramidal trigonal bipyramidal AB5 5 0 AB4E 4 1 10.1

22. # of atoms bonded tocentral atom # lone pairs on central atom trigonal bipyramidal distorted tetrahedron Arrangement ofelectron pairs Molecular Geometry AB4E 4 1 Class trigonal bipyramidal T-shaped F F Cl F VSEPR trigonal bipyramidal trigonal bipyramidal AB5 5 0 AB3E2 3 2 10.1

23. # of atoms bonded tocentral atom # lone pairs on central atom trigonal bipyramidal distorted tetrahedron Arrangement ofelectron pairs Molecular Geometry AB4E 4 1 Class trigonal bipyramidal T-shaped AB3E2 3 2 trigonal bipyramidal linear I I I VSEPR trigonal bipyramidal trigonal bipyramidal AB5 5 0 AB2E3 2 3 10.1

24. octahedral octahedral AB6 6 0 # of atoms bonded tocentral atom # lone pairs on central atom square pyramidal octahedral Arrangement ofelectron pairs Molecular Geometry Class F F F Br F F VSEPR AB5E 5 1 10.1

25. octahedral octahedral AB6 6 0 # of atoms bonded tocentral atom # lone pairs on central atom square pyramidal octahedral AB5E 5 1 Arrangement ofelectron pairs Molecular Geometry Class square planar octahedral F F Xe F F VSEPR AB4E2 4 2 10.1

26. What are the molecular geometries of SO2 and SF4? F S F F O O S F Predicting Molecular Geometry • Draw Lewis structure for molecule. • Count number of lone pairs on the central atom and number of atoms bonded to the central atom. • Use VSEPR to predict the geometry of the molecule. AB4E AB2E distorted tetrahedron bent 10.1

27. Examples / Practice: • Draw Lewis structures, build models, and predict the shape of the following molecules: • Carbon tetrahydride • Boron Trichloride • Carbon dioxide (carbons and oxygens double bonded to each other) • Selenium dibromide • Water • Nitrogen trifluoride