Question

draw the molecular orbital (mo) electron diagram for the he₂⁻ molecular ion. be sure your diagram contains all of the electrons in the ion, including any core electrons.

Explanation:

Step1: Determine number of electrons

Helium has 2 electrons per atom. The $\text{He}_2^{-}$ ion has $2\times2 + 1=5$ electrons.

Step2: Fill molecular - orbitals

The molecular - orbital energy order for $\text{He}_2^{-}$ is $\sigma_{1s}<\sigma_{1s}^*$. First, fill the lower - energy $\sigma_{1s}$ orbital with 2 electrons (spin - paired, one up and one down). Then fill the $\sigma_{1s}^*$ orbital with 3 electrons (2 spin - paired and 1 unpaired).

Answer:

Draw a diagram with two horizontal lines representing the $\sigma_{1s}$ and $\sigma_{1s}^*$ molecular orbitals. Label the $\sigma_{1s}$ orbital at a lower energy level and the $\sigma_{1s}^*$ at a higher energy level. Place two electrons (one up - arrow and one down - arrow) in the $\sigma_{1s}$ orbital and three electrons (two up - down paired and one up - arrow) in the $\sigma_{1s}^*$ orbital.