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