Molecular Orbitals

An overview

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.

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.

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

The Hydrogen Molecule

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

Bonding and Antibonding Orbitals

MO Diagrams

H2 and He2

Complex MO Diagrams

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

0 No bond exists

1 Single bond

2 Double bond

3 Triple bond

Calculate Bond Order for He2+

Compare bond orders

Which would be more stable: He2 or He2+?

Justify your answer.