Chapter 9 - Covalent Bonding: Orbitals

9.1 Hybridization and the Localized Electron Model

• The arrangement of valence electrons is represented by the Lewis structure and the molecular geometry can be predicted from the VSEPR model
• The valence orbitals are the orbitals associated with the highest principal quantum level that contains electrons on a given atom
• Hybridization is a modification of the localized electron model to account for the observation that atoms often seem to use special atomic orbitals in forming molecules
• sp3 hybridization gives a tetrahedral set of orbitals
  • Whenever a set of equivalent tetrahedral atomic orbitals is required by an atom, this model assumes that the atom adopts a set of sp3 orbitals; the atom becomes sp3 hybridized.
• A double bond acts as one effective electron pair
• Whenever an atom is surrounded by three effective pairs, a set of sp2 hybrid orbitals is required.
• Two effective pairs around an atom will always require sp hybridization of that atom
• More rigorous theoretical models of CO2 indicate that each of the oxygen atoms uses two p orbitals simultaneously to form the pi bonds to the carbon atom, thus leading to unusually strong CPO bonds.
• A set of five effective pairs around a given atom always requires a trigonal bipyramidal arrangement, which in turn requires d2 sp3 hybridization of that atom
  • D2 sp3 hybridization gives six orbitals arranged octahedrally.
• In applying the localized electron model, we must remember not to overemphasize the characteristics of the separate atoms
• In the same vein, it is not the orbitals in the isolated atom that matter, but which orbitals the molecule requires for minimum energy

9.2 The Molecular Orbital Model

• Molecular orbitals have many of the same characteristics as atomic orbitals
• Two of the most important are that they can hold two electrons with opposite spins and that the square of the molecular orbital wave function indicates electron probability
• The electron probability of both molecular orbitals is centered along the line passing through the two nuclei
• In the molecule, only the molecular orbitals are available for occupation by electrons
• MO1 is lower in energy than the 1s orbitals of free hydrogen atoms, while MO2 is higher in energy than the 1s orbitals
• Bonding molecular orbital: Lower in energy than the atomic orbitals of which it is composed
• Anti bonding molecular orbital: Higher in energy than the atomic orbitals of which it is composed
• Bonding will result if the molecule has lower energy than the separated atoms
• To indicate bond strength, we use the concept of bond order. Bond order is the difference between the number of bonding electrons and the number of anti-bonding electrons divided by 2.
  • Bond order is an indication of bond strength because it reflects the difference between the number of bonding electrons and the number of antibonding electrons
  • Larger bond order means greater bond strength

9.3 Bonding in Homonuclear Diatomic Molecules

• To participate in molecular orbitals, atomic orbitals must overlap in space
• Paramagnetism causes the substance to be attracted into the inducing magnetic field
  • Studies have shown that paramagnetism is associated with unpaired electrons and diamagnetism is associated with paired electrons
  • Diamagnetism causes the substance to be repelled from the inducing magnetic field.
• There are definite correlations between bond order, bond energy, and bond length
  • As the bond order predicted by the molecular orbital model increases, the bond energy increases, and the bond length decreases
• Comparison of the bond energies of the B2 and F2 molecules indicates that bond order cannot automatically be associated with a particular bond energy
• The molecular orbital model correctly predicts oxygen’s paramagnetism, while the localized electron model predicts a diamagnetic molecule

9.4 Bonding in Heteronuclear Diatomic Molecules

• ****When the two atoms of a diatomic molecule are very different, the energy level diagram for homonuclear molecules can no longer be used
  • A new diagram must be devised for each molecule.
• The molecular orbital containing the bonding electron pair shows greater electron probability close to the fluorine
• This causes the fluorine atom to have a slight excess of negative charge and leaves the hydrogen atom partially positive.
• Thus, the molecular orbital model accounts in a straightforward way for the different electronegativities of hydrogen and fluorine and the resulting unequal charge distribution

9.5 Combining the Localized Electron and Molecular Orbital Moles

• Even with resonance included, the localized electron model does not describe molecules and ions
• It is really the bond that has different locations in the various resonance structures.
• In molecules that require resonance, it is the bonding that is most clearly delocalized.
• Molecules that require the concept of resonance in the localized electron model can be more accurately described by combining the localized electron and molecular orbital models

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