Introduction

• Molecular orbital theory: a theory of chemical bonding in which electrons in molecules occupy molecular orbitals that extend over the entire molecule and are formed by the combination of the atomic orbitals that make up the molecule.

  • Begins with the hypothesis that electrons in atoms exist in atomic orbitals and assumes that electrons in molecules exist in molecular orbitals.

  • Just as the Schrödinger equation can be used to calculate the energies and shapes of atomic orbitals, molecular orbital theory assumes that the Schrödinger equation can also be used to calculate the energies and shapes of molecular orbitals.

  • Molecular orbitals extend over entire molecules because all of the orbitals of all of the atoms take part in constructing molecular orbitals

  • The molecular orbitals are created by the in-phase and out-of-phase addition (sometimes called subtraction) of all the atomic orbitals that are aligned to overlap on all atoms in a molecule.

  • Bonds are delocalized over several atoms

Rules Used in Applying MOT to the Formation of Covalent Bonds

• The number of molecular orbitals formed is equal to the number of atomic orbitals combined

• Just like atomic orbitals, molecular orbitals are arranged in order of increasing energy.

• Filling of molecular orbitals with electrons is governed by the same principles as the filling of atomic orbitals

Bonding and Antibonding

• Bonding molecular orbital: an orbital in which electrons have a lower energy than they would in the isolated atomic orbitals.

  • Formed from the addition of atomic orbitals

• Sigma bonding molecular orbital: an orbital in which electron density lies between the two nuclei, along the axis joining them, and is cylindrically symmetric about the axis

• Antibonding molecular orbital: an orbital in which the electrons in it have a higher energy (are more easily removed) than they would have in the isolated atomic orbitals.

  • Population of this orbital with electrons actually causes repulsion of the nuclei involved.

  • Formed from the subtraction of atomic orbitals

• The ground state of an atom or a molecule is its state of lowest energy

• An excited state is any electronic state other than the ground state