How many 3d electrons are present in the $ ext{Fe}^{3+}$ ion, and what is its expected distribution

Question

How many 3d electrons are present in the $	ext{Fe}^{3+}$ ion, and what is its expected distribution in the five 3d orbitals in a weak octahedral field (like water)?

Professor PC Thomas Classes · Answer

Iron (Z=26) is $3d^6 4s^2$. $	ext{Fe}^{3+}$ is $3d^5$. In a weak octahedral field like water, the pairing energy is high, so the electrons occupy all five orbitals singly before pairing according to Hund's rule, resulting in a $t_{2g}^3 e_g^2$ configuration.