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

1 Answer
Jul 3, 2017

#(sigma_(2s))^2 (sigma_(2s)^"*")^2 (sigma_(2p_z))^2 (pi_(2p_x))^2 (pi_(2p_y))^2 (pi_(2p_x)^"*")^2 (pi_(2p_y)^"*")^2#


Recall that there are orbital mixing effects for homonuclear diatomic molecules that decrease from left to right until #"N"_2# (inclusive), which gives rise to a molecular orbital ordering of #(pi_(2p_x), pi_(2p_y))# and then #sigma_(2p_z)#, upwards. See here if you don't remember.

Inorganic Chemistry, Miessler et al., Ch. 5.2.3

Notice how the #sigma_(g)(2p)#, or the #sigma_(2p_z)# molecular orbital, dips down below the #pi# molecular orbital energies after #"N"_2#.

Since #"F"_2# is after #"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_(2s)# molecular orbitals are significantly lower in energy than #sigma_(2p_z)# molecular orbitals because the corresponding #2s# atomic orbitals are significantly lower in energy than the #2p#'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 #ns//ns# overlap for a homonuclear diatomic molecule gives rise to a partial MO diagram like this:

and an #np//np# overlap for #"O"_2# and #"F"_2# gives:

So, the full MO diagram is:

http://www.grandinetti.org/

Thus, the valence electron configuration is:

#color(blue)((sigma_(2s))^2 (sigma_(2s)^"*")^2 (sigma_(2p_z))^2 (pi_(2p_x))^2 (pi_(2p_y))^2 (pi_(2p_x)^"*")^2 (pi_(2p_y)^"*")^2)#