1. Molecular Orbitals An overview

2. MO Theory Robert Mullikan won the Nobel Prize in 1966 for developing this theory. This theory describes the electrons in orbitals belonging to the entire molecule.

3. Molecular Orbitals A molecular orbital (MO) can hold a maximum of two electrons. A MO has a definite energy. We can represent an MO with an electron- density cloud.

4. LCAO Linear Combination of Atomic Orbitals Whenever two atomic orbitals overlap, two molecular orbitals form. # in = # out Energy is conserved, so… One orbital will be lower in energy One orbital will be higher in energy

5. The Hydrogen Molecule

6. The Two Orbitals  * Antibonding orbital Very little electron density between the nuclei Higher in energy Destabilizes bond formation  Bonding orbital Promotes bond formation Electron density is between the nuclei Lower in energy

7. Bonding and Antibonding Orbitals

8. MO Diagrams

9. H 2 and He 2

10. Complex MO Diagrams

11. Bond Order In MO theory, bond stability of a covalent bond is related to its bond order. Bond order = ½(# bonding electrons - # antibonding electrons) Bond order can be an integer or a fraction Bond Order # of bonds 0No bond exists 1Single bond 2Double bond 3Triple bond

12. Calculate Bond Order for He 2 +

13. Compare bond orders Which would be more stable: He 2 or He 2 + ? Justify your answer.