Building the Periodic Table — Question 3

Question 3

Removing outer-shell electrons need not reverse the neutral filling mnemonic

Use the supplied neutral iron configuration $[\mathrm{Ar}]3d^64s^2$, with an 18-electron argon core. For the ions considered here, remove 4s electrons before 3d electrons. Treat the d subshell with Hund filling and neglect ligand fields.

1. Write configurations and electron counts for $\mathrm{Fe}^{2+}$ and $\mathrm{Fe}^{3+}$.

2. Find their numbers of unpaired electrons and total-spin values in this model.

Question 3 - Solution

Removing outer-shell electrons need not reverse the neutral filling mnemonic

(a) Method and calculation

$$\boxed{\mathrm{Fe}^{2+}:[\mathrm{Ar}]3d^6,\quad N_e=24}.$$$$\boxed{\mathrm{Fe}^{3+}:[\mathrm{Ar}]3d^5,\quad N_e=23}.$$

(b) Interpretation and further calculation

$$d^6:\ (\uparrow\downarrow)(\uparrow)(\uparrow)(\uparrow)(\uparrow)\Rightarrow\boxed{N_u=4,\ S=2}.$$$$d^5:\ (\uparrow)(\uparrow)(\uparrow)(\uparrow)(\uparrow)\Rightarrow\boxed{N_u=5,\ S=5/2}.$$

Physical check

Ionization changes the self-consistent orbital energies. The rule that 4s is occupied early in a neutral-atom mnemonic does not imply that it is retained longest during ionization.