Question

What is the molecular orbital diagram for `B_2`?

Answer 1

Before we can draw a molecular orbital diagram for B₂, we must find the in-phase and out-of-phase overlap combinations for boron's atomic orbitals.

Then we rank them in order of increasing energy.

We can ignore the `1s` orbitals, because they do not contain the valence electrons.

Each boron atom has one `2s` and three `2p` valence orbitals.

The `2s` orbitals will overlap to form `2sσ` and `2sσ"*"` orbitals.

The `2p_x` orbitals can overlap end-on to form `2pσ` and `2pσ"*"` orbitals.

The `2p_y` and `2p_z` orbitals can overlap side-on to form degenerate pairs of `2pπ` and `2pπ"*"` orbitals.

The order of energies is `2sσ < 2sσ^✳ < 2pπ < 2pσ < 2pπ^✳ < 2pσ^✳`.

We have three valence electrons from each B atom, so B₂ will have six valence electrons.

We use the Pauli Exclusion Principle and Hund's rule to fill the orbitals in an Aufbau process.

The molecular orbital diagram for B₂ then becomes

The video below describes how to generate molecular orbital diagrams for B₂ and other diatomic molecules from Row 2 elements of the Periodic Table.