Ch 2: Bonding

Chapter 2: Bonding

Bonding in H₂(contd)

On the previous two pages we saw first how two waves and then how two 1s orbitals can interact in-phase and out-of phase. Now we will look at how this relates to what happens between the 1s orbitals of the two interacting hydrogen atoms and how this helps us create a view of the situation in a hydrogen molecule:

The atomic orbitals of the two hydrogen atoms, the 1s orbitals, are represented on the outsides by two blue spheres. In the middle are the in-phase and out-of-phase combinations for the molecule.

The in-phase combination is at lower energy than the 1s orbitals we started from and is called the bonding molecular orbital since it is responsible for the electron density between the two nuclei. This molecular orbital is symmetrical with respect to rotation about an axis on which both H nuclei lie (the internuclear axis), this is termed a "sigma" orbital i.e. σ-orbital.

The out-of-phase combination is at higher energy than the 1s orbitals we started from. This orbital is called the anti-bonding molecular orbital for reasons we will talk about shortly. It is also symmetrical with respect to rotation about an axis on which both H nuclei lie, so it is also a σ-orbital, but because it is the out-of-phase combination, it is termed a "sigma-star" orbital i.e. σ* -orbital.

H
atomic
orbital

H₂
molecular
orbital

H
atomic
orbital

Now for the electrons.... the electrons are "placed" in the molecular orbitals following the same rules as for filling orbitals in atoms (i.e. lowest energy first). This means the two 1s electrons (one from each H atom) both go into the bonding molecular orbital, this results in stabilisation of the system (the combined two electrons in the H-H molecule are at lower energy than the two individual electrons of the separate H atoms. Hence, two H atoms combine to become more stable as a H₂ molecule.

IMPORTANT: Only orbitals containing electrons contribute to the stability of the molecule, so the empty σ^*-orbital has no impact on the stability of the molecule in this example.