In magnetite, $Fe X_{3} O X_{4}$ (an inverse spinel), the tetrahedral sites hold one high-spin $Fe X^{3 +}$ and the octahedral sites hold one $Fe X^{2 +}$ and one $Fe X^{3 +}$ per formula unit. The moments of the tetrahedral sublattice are antiparallel to those of the octahedral sublattice. The net spin-only moment per formula unit, counted as the number of unpaired electrons, is

Answer: D

Unpaired electrons:

Fe X^{3 +}

(

d^{5}

) 5 and

Fe X^{2 +}

(

d^{6}

, high spin) 4. Octahedral sites:

5 + 4 = 9

aligned together; tetrahedral

Fe X^{3 +}

: 5 aligned oppositely. Net

= 9 - 5 = 4

unpaired electrons per formula unit. If all were parallel one would get 14.