Chapter 10 Molecular Geometry and Bonding Theory

Molecular Geometry and Directional Bonding

VSEPR Theory

Dipole Moment and Molecular Geometry

Hybrid Orbitals- Valence Bond Theory-VBT

Multiple Bonds- Valence Bond Theory

Molecular OrbitalTheory

Principles of M.O. Theory

Second-Period Diatomic Molecules

MO's and Delocalized Bonding

Objectives

• Use VSEPR theory to predict the geometry around any atom in any molecule or ion.
• explain why unshared electron pairs exert a greater repulsive interaction than do shared pairs.
• predict whether a molecule has a dipole moment.
• use Valence Bond hybrid orbital theory to assign a hybrid orbital set to an atom.
• give the geometry (including approximate bond angles) and hybridization on any atom in any molecule or ion.
• describe the bonding in a molecule or ion in terms of sigma (s) and pi (p) bonds, from Valence Bond Theory.
• explain the concept of delocalization in pi (p) bonding.
• use an M.O. diagram to predict bonding and paramagnetism or diamagnetism in simple diatomic molecules.
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Molecular Geometry and Directional Bonding

VSEPR Theory--(Valence Shell Electron Pair Repulsion Theory)

• Draw the Lewis dot structure
• Look at lone pairs and double bonds on molecules
• Electron pairs repel each other
• lone pairs repel more than bonded pairs. Why?
• Double bonds repel more than single bonds. Why?
• Look at tables in book.

Dipole Moment and Molecular Geometry

To determine if a molecule has a dipole moment (is polar) consider electronegativity and geometry.

1. A molecule must have polar bonds.
2. A molecule must be NON symmetric. In other words the partial positive, d+ and d- charges must be on opposite sides of the molecule.

Hybrid Orbitals- Valence Bond Theory

Combinations of outer atomic orbitals, s, p, d, result in a new set of orbitals that are some combination of the atomic orbitals (see text)

Hybidized C in CH₄

NH₃ as another example

XeF₄

In summary:

• Draw the Lewis dot structure
• Look at lone prs and double bonds
• Determine the number of p and s bonds and lone pairs on the atom of interest
  • 2 regions = sp
  • 3 regions = sp²
  • 4 regions = sp³
  • 5 regions = sp³d
  • 6 regions = sp³d²

Single, double, triple bonds and lone pairs each count as 1 region.

• Multiple Bonds- Valence Bond Theory

See text diagrams- ethylene and acetylene

The second bond is a p- orbital overlap (pi, p, bond)

SO₂ example

Summary Example

1. Write the Lewis Dot Structure for CH₂O

2. Determine the e^- pair geometry and molecular shape.

3. Is it polar? Why?

4. Determine the hybridization of the central C and the O atom

5. Describe the bonding in the structure --

p, s, cov., p. cov., etc. draw a diagram of the electron density regions.

Molecular OrbitalTheory

Principles of M.O. Theory

• Useful in explaining some aspects of bonding, eg.: predicts formation of diatomic molecules, paramagnetism, and bond order.
• Uses Linear Combinations of Atomic Orbitals to construct Molecular Orbitals.
• Second-Period Diatomic Molecules

MO's and Delocalized Bonding

Be able to:

1. fill in the table given the order of filling of the MO's
2. Calculate the bond order given the MO table or listing.
3. Tell whether or not the molecule is diamagnetic or paramagnetic.

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Updated Feb. 13, 2004. Questions or comments on this Web site should go to Robin Terjeson.