Question

How many unpaired electrons are present in Fe$^{+3}$, Zn$^{+2}$ and Mn$^{+2}$ ?

Hint:

Half-filled (d$^5$) and fully-filled (d$^{10}$) shells are extra stable. Both Fe$^{+3}$ and Mn$^{+2}$ are d$^5$.

Answer 1

1. Fe$^{+3}$ (Iron, Z=26):
Neutral Fe: [Ar] 3d$^6$ 4s$^2$.
Fe$^{+3}$ (remove 2 from s, 1 from d): [Ar] 3d$^5$.
Hund's rule: 5 electrons in 5 d-orbitals $\implies$ 5 unpaired electrons.
2. Zn$^{+2}$ (Zinc, Z=30):
Neutral Zn: [Ar] 3d$^{10}$ 4s$^2$.
Zn$^{+2}$ (remove 2 from s): [Ar] 3d$^{10}$.
All electrons paired $\implies$ 0 unpaired electrons.
3. Mn$^{+2}$ (Manganese, Z=25):
Neutral Mn: [Ar] 3d$^5$ 4s$^2$.
Mn$^{+2}$ (remove 2 from s): [Ar] 3d$^5$.
Hund's rule: 5 electrons in 5 d-orbitals $\implies$ 5 unpaired electrons.