Question

{"given":"MgO crystallizes in a rock salt (NaCl) type unit cell structure.","key_observation":"In a rock salt (NaCl) type structure, the coordination number of both the cation and the anion is 6. This means each cation is surrounded by 6 anions, and each anion is surrounded by 6 cations.","formula_steps":[],"option_analysis":[{"label":"(A)","text":"6 $\\text{Mg}^{2+}$ ions","verdict":"incorrect","explanation":"Cations are surrounded by anions in the first coordination sphere, not by other cations."},{"label":"(B)","text":"6 $\\text{O}^{2-}$ ions","verdict":"correct","explanation":"In a rock salt structure, the coordination number is 6. Therefore, each $\\text{Mg}^{2+}$ cation is in direct contact with 6 $\\text{O}^{2-}$ anions."},{"label":"(C)","text":"8 $\\text{O}^{2-}$ ions","verdict":"incorrect","explanation":"A coordination number of 8 corresponds to a body-centered cubic (CsCl type) structure, not a rock salt structure."},{"label":"(D)","text":"4 $\\text{O}^{2-}$ ions","verdict":"incorrect","explanation":"A coordination number of 4 corresponds to a tetrahedral (ZnS type) structure, not a rock salt structure."}],"answer":"(B)"}