The Molecular Orbitals of the Hydrogen Molecule

Combining the 1s orbitals of each hydrogen atom using LCAO, two molecular orbitals are generated σ_1s (pronounced sigma one s) and σ*_1s (pronounced sigma star one s).

The σ_1s orbital is generated by a constructive combination (or interference), where the two atomic orbitals wave functions reinforce (add to) each other. This is the lower energy of the two molecular orbitals and is known as the bonding molecular orbital. Notice in Figure 1.1 “Hydrogen molecular orbital combination diagram” that the electron density of this orbital is concentrated between the two nuclei. These electrons are stabilized by attractions to both nuclei, and they hold the atoms together with a covalent bond.

The σ*_1s orbital is generated by a destructive combination (or interference), where the wave functions of the two atomic orbitals cancel each other. This type of combination results in an area of zero electron density between the two nuclei, known as a nodal plane (or node). This node of zero electron density is destabilizing toward the bond, making it higher energy, and subsequently this type of orbital is known as an antibonding molecular orbital (denoted by the asterisk in the orbital name).

Figure 1.1. Hydrogen molecular orbital combination diagram

Similar to atomic orbitals, we can write electron configuration energy diagrams for molecular orbitals (Figure 1.2 “Hydrogen molecular orbital electron configuration energy diagram”). Notice that the atomic orbitals of each atom are written on either side, and the newly formed molecular orbitals are written in the centre of the diagram. The bonding molecular orbital is filled and is relatively lower in energy than the contributing atomic orbitals, supporting the fact that hydrogen molecules (H₂) are more stable than lone hydrogen atoms.

Figure 1.2. Hydrogen molecular orbital electron configuration energy diagram.