Question

draw the molecular orbital (mo) electron diagram for the be₂⁻ molecular ion. be sure your diagram contains all of the electrons in the ion, including any core electrons.

Explanation:

Step1: Determine number of electrons

Beryllium (Be) has an atomic number of 4, so each Be atom has 4 electrons. The $\text{Be}_{2}^{-}$ ion has $2\times4 + 1=9$ electrons.

Step2: Set up MO diagram framework

We start with the molecular - orbital energy - level diagram for a homonuclear diatomic molecule. The lowest - energy molecular orbitals are the $\sigma_{1s}$, $\sigma_{1s}^*$, $\sigma_{2s}$, $\sigma_{2s}^*$, $\pi_{2p_x}=\pi_{2p_y}$, $\sigma_{2p_z}$, $\pi_{2p_x}^*=\pi_{2p_y}^*$, $\sigma_{2p_z}^*$ in order of increasing energy.

Step3: Fill in electrons

We fill the molecular orbitals with electrons according to the Aufbau principle (fill lower - energy orbitals first), Pauli exclusion principle (each orbital can hold a maximum of 2 electrons with opposite spins), and Hund's rule (when filling degenerate orbitals, electrons first occupy them singly with parallel spins).

• Fill $\sigma_{1s}$ with 2 electrons.
• Fill $\sigma_{1s}^*$ with 2 electrons.
• Fill $\sigma_{2s}$ with 2 electrons.
• Fill $\sigma_{2s}^*$ with 2 electrons.
• The remaining 1 electron goes into the $\pi_{2p_x}$ (or $\pi_{2p_y}$) orbital.

Answer:

The molecular - orbital diagram will have the following orbitals filled: $\sigma_{1s}$ (2 electrons), $\sigma_{1s}^*$ (2 electrons), $\sigma_{2s}$ (2 electrons), $\sigma_{2s}^*$ (2 electrons), $\pi_{2p_x}$ (1 electron). Draw the energy - level diagram with these orbitals labeled on the y - axis (energy), and the electrons represented as arrows (spin - up or spin - down) in the appropriate boxes for each orbital.