Which of the following correctly represents the molecular orbital energy diagram for the diatomic molecule Li_2?

• A. The diagram shows two electrons occupying the sigma_{2s} bonding orbital, with no electrons in the sigma_{2s}^* antibonding orbital.
• B. The diagram shows one electron in the sigma_{1s} bonding orbital and one electron in the sigma_{1s}^* antibonding orbital.
• C. The diagram shows both electrons occupying the sigma_{2s}^* antibonding orbital.
• D. The diagram shows one electron in the sigma_{2s} bonding orbital and one electron in the sigma_{2s}^* antibonding orbital.

Answer: (A)

Verified step by step guidance

1. Identify the total number of valence electrons in the Li_2 molecule. Each lithium atom has 3 electrons, with 1 valence electron in the 2s orbital, so Li_2 has 2 valence electrons in total.
2. Recall the order of molecular orbitals for homonuclear diatomic molecules of period 2 elements like Li_2, focusing on the 2s orbitals: the bonding orbital is \$\sigma_{2s}\$ and the antibonding orbital is \$\sigma_{2s}^*\$.
3. Place the 2 valence electrons into the molecular orbitals starting from the lowest energy level, which is the bonding \$\sigma_{2s}\$ orbital, following the Pauli exclusion principle and Hund's rule.
4. Since there are only 2 electrons, both will occupy the \$\sigma_{2s}\$ bonding orbital, and the \$\sigma_{2s}^*\$ antibonding orbital remains empty.
5. Conclude that the correct molecular orbital diagram for Li_2 shows two electrons in the \$\sigma_{2s}\$ bonding orbital and none in the \$\sigma_{2s}^*\$ antibonding orbital.