Q. 264.5( 2 Votes )
In which of the following structures coordination number for cations and anions in the packed structure will be same?
A. Cl– ion form fcc lattice and Na+ ions occupy all octahedral voids of the unit cell.
B. Ca2+ ions form fcc lattice and F– ions occupy all the eight tetrahedral voids of the unit cell.
C. O2– ions form fcc lattice and Na+ ions occupy all the eight tetrahedral voids of the unit cell.
D. S2– ions form fcc lattice and Zn2+ ions go into alternate tetrahedral voids of the unit cell.
In NaCl, the Cl- ions are arranged in ccp arrangement where Cl- ions are present at the corners as well as the center of the face of the cube. The Na+ ions are located such that each Na+ ion is surrounded with six chloride ions. Na+ ions occupy all the octahedral voids. Each chloride ion is also surrounded by six Na+ ions and the stoichiometry is 1:1. Thus the coordination number for both species is the same.
In CaF2 arrangement, the Ca2+ ions are arranged in ccp arrangement where Ca2+ ions are placed at the corners as well as the center of the face of the cube. Fluoride ions occupy tetrahedral voids and in way that each ion is surrounded by four calcium ions. The coordination number of F- is 4. There are two tetrahedral sites available for each calcium ion, and thus the F- ions occupy all eight tetrahedral voids. However, the stoichiometry is 1:2, and the coordination number of calcium ion is 8, therefore the coordination number for these species is not the same.
In Na2O arrangement, the O2- ions are arranged in ccp arrangement where O2- ions are placed at corners as well as the center of the face of the cube. Na+ ions occupy all tetrahedral voids surrounding O2- ion. Each sodium ion is surrounded by 4 oxide ions and each oxide ion is surrounded by 8 sodium ions. The coordination number of Na+ is 4 and O2- is 8, therefore the coordination number of these species is not the same.
In Zinc blende ZnS arrangement, the S2- atoms are in ccp arrangement, where each sulphideion is surrounded by four zinc ions. There are eight tetrahedral voids and four sulphide ions occupy half of the tetrahedral holes. Each sulphide ion itself is surrounded by four zinc ions, thus making both of their coordination numbers 4.
Thus the correct answer is (i).
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