Question

{"given":"The crystal structure of calcium fluoride ($CaF_2$), also known as the fluorite structure.","key_observation":"In the fluorite ($CaF_2$) structure, the $Ca^{2+}$ ions form a face-centered cubic (fcc) lattice, while the $F^-$ ions occupy all the tetrahedral voids.","formula_steps":[],"option_analysis":[{"label":"(A)","text":"8 and 4","verdict":"correct","explanation":"Since $Ca^{2+}$ ions form the fcc lattice, each $Ca^{2+}$ ion is surrounded by 8 $F^-$ ions located in the tetrahedral voids, giving it a coordination number of 8. Each $F^-$ ion occupies a tetrahedral void, meaning it is surrounded by 4 $Ca^{2+}$ ions, giving it a coordination number of 4."},{"label":"(B)","text":"6 and 8","verdict":"incorrect","explanation":"This does not match the fluorite structure. A coordination of 6:3 is seen in rutile ($TiO_2$), but 6:8 is not applicable here."},{"label":"(C)","text":"4 and 4","verdict":"incorrect","explanation":"A 4:4 coordination number is characteristic of the zinc blende ($ZnS$) structure, not the fluorite structure."},{"label":"(D)","text":"4 and 8","verdict":"incorrect","explanation":"A 4:8 coordination number corresponds to the anti-fluorite structure (e.g., $Na_2O$), which is the exact reverse of the fluorite structure."}],"answer":"(A)"}