Understanding Ne₂ Molecular Orbitals

To determine the Highest Occupied Molecular Orbital (HOMO) for $Ne_2$, we need to analyze its molecular orbital (MO) configuration.

Neon Atomic Configuration

Neon (Ne) is element number 10. Its electron configuration is $1s^2 2s^2 2p^6$. For molecular orbital formation, we focus on the valence electrons, which are in the 2s and 2p subshells.

Ne₂ Molecular Orbitals

A $Ne_2$ molecule is formed from two Neon atoms, resulting in a total of $2 \times 8 = 16$ electrons. The molecular orbitals are formed by combining the atomic orbitals. For elements like Neon, the energy order of molecular orbitals derived from 2s and 2p atomic orbitals is typically:

$\sigma_{2s}, \sigma^*_{2s}, \pi_{2p}, \sigma_{2p}, \pi^*_{2p}, \sigma^*_{2p}$

Note: The $\pi_{2p}$ orbitals are doubly degenerate, and the $\pi^*_{2p}$ orbitals are also doubly degenerate.

Filling Molecular Orbitals

We fill these molecular orbitals with the 16 total electrons according to the Pauli Exclusion Principle and Hund's rule:

• $\sigma_{2s}$: 2 electrons
• $\sigma^*_{2s}$: 2 electrons
• $\pi_{2p}$: 4 electrons (2 in each degenerate orbital)
• $\sigma_{2p}$: 2 electrons
• $\pi^*_{2p}$: 4 electrons (2 in each degenerate orbital)
• $\sigma^*_{2p}$: 2 electrons

The complete electron configuration is: $\sigma_{2s}^2 \sigma^*_{2s}^2 \pi_{2p}^4 \sigma_{2p}^2 \pi^*_{2p}^4 \sigma^*_{2p}^2$.

Identifying the HOMO

The Highest Occupied Molecular Orbital (HOMO) is the last molecular orbital filled with electrons. In the $Ne_2$ configuration, the last filled orbital is $\sigma^*_{2p}$.

Therefore, the HOMO for $Ne_2$ is $\sigma^*_{2p}$.