# What is the molecular electron configuration of "F"_2?

Jul 3, 2017

${\left({\sigma}_{2 s}\right)}^{2} {\left({\sigma}_{2 s}^{\text{*")^2 (sigma_(2p_z))^2 (pi_(2p_x))^2 (pi_(2p_y))^2 (pi_(2p_x)^"*")^2 (pi_(2p_y)^"*}}\right)}^{2}$

Recall that there are orbital mixing effects for homonuclear diatomic molecules that decrease from left to right until ${\text{N}}_{2}$ (inclusive), which gives rise to a molecular orbital ordering of $\left({\pi}_{2 {p}_{x}} , {\pi}_{2 {p}_{y}}\right)$ and then ${\sigma}_{2 {p}_{z}}$, upwards. See here if you don't remember.

Notice how the ${\sigma}_{g} \left(2 p\right)$, or the ${\sigma}_{2 {p}_{z}}$ molecular orbital, dips down below the $\pi$ molecular orbital energies after ${\text{N}}_{2}$.

Since ${\text{F}}_{2}$ is after ${\text{N}}_{2}$ in the second row of the periodic table (where these effects are not present), the orbital energy ordering is "normal".

In general, the molecular orbital energies follow these rules:

• The relative atomic orbital energy differences approximate the relative $\sigma / \sigma$ orbital energy differences.

So, ${\sigma}_{2 s}$ molecular orbitals are significantly lower in energy than ${\sigma}_{2 {p}_{z}}$ molecular orbitals because the corresponding $2 s$ atomic orbitals are significantly lower in energy than the $2 p$'s.

• $\sigma$ molecular orbitals are singly-degenerate, and $\pi$ molecular orbitals are doubly-degenerate.
• $\sigma$ molecular orbitals, in principle, get more stabilized upon overlap than $\pi$ molecular orbitals do.

For example, an $n s / n s$ overlap for a homonuclear diatomic molecule gives rise to a partial MO diagram like this:

and an $n p / n p$ overlap for ${\text{O}}_{2}$ and ${\text{F}}_{2}$ gives:

So, the full MO diagram is:

Thus, the valence electron configuration is:

$\textcolor{b l u e}{{\left({\sigma}_{2 s}\right)}^{2} {\left({\sigma}_{2 s}^{\text{*")^2 (sigma_(2p_z))^2 (pi_(2p_x))^2 (pi_(2p_y))^2 (pi_(2p_x)^"*")^2 (pi_(2p_y)^"*}}\right)}^{2}}$