Which of the following correctly describes the molecular orbital energy diagram for the diatomic molecule H_2?

• A. Both electrons are unpaired and occupy separate atomic orbitals.
• B. Two electrons occupy the bonding sigma_{1s} orbital, and the antibonding sigma_{1s}^* orbital remains empty.
• C. Both electrons occupy the antibonding sigma_{1s}^* orbital.
• D. One electron occupies the bonding sigma_{1s} orbital, and one electron occupies the antibonding sigma_{1s}^* orbital.

Answer: (B)

Verified step by step guidance

1. Recall that the hydrogen molecule (H_2) consists of two hydrogen atoms, each contributing one 1s electron, for a total of two electrons.
2. Understand that when atomic orbitals combine to form molecular orbitals, the 1s orbitals combine to form a lower-energy bonding orbital (\sigma_{1s}) and a higher-energy antibonding orbital (\sigma_{1s}^*).
3. Apply the Aufbau principle to fill the molecular orbitals: electrons fill the lowest energy orbitals first, so both electrons will occupy the bonding \sigma_{1s} orbital before any occupy the antibonding \sigma_{1s}^* orbital.
4. Recognize that electrons tend to pair up with opposite spins in the same orbital, so the two electrons in the \sigma_{1s} bonding orbital will be paired.
5. Conclude that the correct molecular orbital configuration for H_2 is two electrons paired in the bonding \sigma_{1s} orbital, with the antibonding \sigma_{1s}^* orbital remaining empty.