Question

Use the MO model to explain the bonding in BeH₂. When constructing the MO energy-level diagram, assume that the Be 1s electrons are not involved in bond formation.

Short answer

The molecular orbitals for BeH₂ are formed from the two hydrogen 1s orbitals and the 2s and one of the 2p orbitals from beryllium. Overlapping of hydrogen 1s orbital with a beryllium 2s orbital results in formation of one of the sigma bonds. Let us assume that the linear BeH₂ molecules has the z axis as the internuclear axis. The 2p_z orbital from beryllium has proper symmetry to overlap with the 1s orbitals from hydrogen. But the 2p_x and 2p_y orbitals are nonbonding orbitals. These orbitals don’t have proper symmetry.

Step-by-step solution

Explanation

The molecular orbitals for BeH₂ are formed from the two hydrogen 1s orbitals and the 2s and one of the 2p orbitals from beryllium. Overlapping of hydrogen 1s orbital with a beryllium 2s orbital results in formation of one of the sigma bonds. Let us assume that the linear BeH₂ molecules has the z axis as the internuclear axis. The 2p_z orbital from beryllium has proper symmetry to overlap with the 1s orbitals from hydrogen. But the 2p_x and 2p_y orbitals are nonbonding orbitals. These orbitals don’t have proper symmetry.

Energy-level Diagram

The MO diagram for BeH₂ is as follows:

Bond order = (4 - 0)/2 = 2. From the MO diagram it can be predicted that BeH₂ is the stable species and is diamagnetic.