Solution

The statements A, C and D are correct. (A) If three $$Fe^{2+}$$ ions are missing from their lattice in FeO, there must be two $$Fe^{3+}$$ ions somewhere in the lattice. The charge on three $$Fe^{2+}$$ ions is $$3 \times (+2) = +6$$ The charge on two $$Fe^{3+}$$ ions is $$2 \times{3+} = +6$$ Thus, the electrical neutrality is maintained. Two type of vacancies of iron are present here. This is called crystal with metal deficiency defect. (B) Crystals with metal deficiency defects are not called super conductors. The idea of Superconductors is that it possesses zero resistance at a certain temperature. (C) Crystals with metal deficiency are called semiconductors. (D) Bohr Magneton = $$9.27 \times 10^{-24} A \:m^2$$. The orbital or spin magnetic momentum of an electron is expressed in terms of Bohr magneton. Its symbol is $$\mu _B$$. Except option B all are correct.