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Molecular Shapes

Molecular Shapes. Molecular Shapes. In order to predict molecular shape, we assume the valence electrons repel each other. Therefore, the molecule adopts whichever 3D geometry minimized this repulsion. We call this process V alence S hell E lectron P air R epulsion ( VSEPR ) theory.

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Molecular Shapes

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  1. Molecular Shapes

  2. Molecular Shapes In order to predict molecular shape, we assume the valence electrons repel each other. Therefore, the molecule adopts whichever 3D geometry minimized this repulsion. We call this process Valence Shell Electron Pair Repulsion (VSEPR) theory.

  3. The VSEPR Model – common Mol. Geo.

  4. Electron-Domain Geo.

  5. The VSEPR Model Predicting Molecular Geometries

  6. The VSEPR Model Predicting Molecular Geometries

  7. The VSEPR Model Predicting Molecular Geometries

  8. The VSEPR Model Molecules with Expanded Valence Shells

  9. The VSEPR Model Molecules with Expanded Valence Shells

  10. The VSEPR Model • We determine the electron domain geometry by looking at electrons around the central atom. • We name the molecular geometry by the positions of atoms. • We ignore lone pairs in the molecular geometry.

  11. The VSEPR Model • The Effect of Nonbonding Electrons and Multiple Bonds on Bond Angles • By experiment, the H-X-H bond angle decreases on moving from C to N to O: • Since electrons in a bond are attracted by two nuclei, they do not repel as much as lone pairs. • Therefore, the bond angle decreases as the number of lone pairs increase.

  12. The VSEPR Model The Effect of Nonbonding Electrons and Multiple Bonds on Bond Angles Similarly, electrons in multiple bonds repel more than electrons in single bonds.

  13. The VSEPR Model Molecules with More than One Central Atom In acetic acid, CH3COOH, there are three central atoms. We assign the geometry about each central atom separately.

  14. Polarity of Molecules Polar molecules interact with electric fields. If the centers of negative and positive charge do not coincide, then the molecule is polar.

  15. Polarity of Molecules Dipole Moments of Polyatomic Molecules Example: in CO2, each C-O dipole is canceled because the molecule is linear. In H2O, the H-O dipoles do not cancel because the molecule is bent.

  16. Polarity of Molecules Dipole Moments of Polyatomic Molecules

  17. Covalent Bonding and Orbital Overlap • Lewis structures and VSEPR do not explain why a bond forms. • How do we account for shape in terms of quantum mechanics? • What are the orbitals that are involved in bonding? • We use Valence Bond Theory: • Bonds form when orbitals on atoms overlap. • There are two electrons of opposite spin in the orbital overlap.

  18. Covalent Bonding and Orbital Overlap

  19. Covalent Bonding and Orbital Overlap

  20. Hybrid Orbitals • sp Hybrid Orbitals • Consider the BeF2 molecule (experimentally known to exist): • Be has a 1s22s2 electron configuration. • There is no unpaired electron available for bonding. • We conclude that the atomic orbitals are not adequate to describe orbitals in molecules. • We know that the F-Be-F bond angle is 180 (VSEPR theory). • We also know that one electron from Be is shared with each one of the unpaired electrons from F.

  21. Formation of sp Hybrid Orbital FG09_013.JPG

  22. Formation of sp2 Orbitals FG09_015.JPG

  23. Formation of sp3 Orbitals FG09_016.JPG

  24. Bonding in H2O FG09_017.JPG

  25. Hybridization in Ethylene FG09_021.JPG

  26. Table 9.4p 366 TB09_005.JPG

  27. Hybrid Orbitals • Summary • To assign hybridization: • draw a Lewis structure; • assign the electron pair geometry using VSEPR theory; • from the electron pair geometry, determine the hybridization; and • name the molecular geometry by the positions of the atoms.

  28. Pi Bond Formation in Ethylene FG09_022.JPG

  29. Triple Bond in Acetylene FG09_023.JPG

  30. Bonding in Benzene FG09_028.JPG

  31. Orbitals of Benzene FG09_029.JPG

  32. Molecular Orbitals • Some aspects of bonding are not explained by Lewis structures, VSEPR theory and hybridization. (E.g. why does O2 interact with a magnetic field?; Why are some molecules colored?) For these molecules, we use Molecular Orbital (MO) Theory. Just as electrons in atoms are found in atomic orbitals, electrons in molecules are found in molecular orbitals. Molecular orbitals: • each contain a maximum of two electrons; • have definite energies; • can be visualized with contour diagrams; • are associated with an entire molecule.

  33. Molecular Orbitals The Hydrogen Molecule

  34. MO Electron Configurations FG09_039.JPG

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