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Question

Which of the following are TRUE for ZnS type and CaF2 type structure?


A

ZnS – 6:6 coordination; CaF2 – 8:4 coordination

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B

ZnS – 4:4 coordination; CaF2 – 8:4 coordination

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C

ZnSZn2+ at half of tetrahedral voids; CaF2 – F- ions are at all tetrahedral voids

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D

ZnSZn2+ at all of tetrahedral voids; CaF2 – F- ions are at all tetrahedral voids

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Solution

The correct option is C

ZnSZn2+ at half of tetrahedral voids; CaF2 – F- ions are at all tetrahedral voids


Just recall the structure of CaF2 here. CaF2 is an example of fluorite type structure only.

The calcium ions, Ca2+, are located at the face-centered cubic lattice points and, therefore, have cubic closed-packed arrangement. The fluoride ions (F ions) occupy all the 8 tetrahedral voids. Thus, there are 4 Ca2+ ions and 8 F ions in the unit cell of calcium fluoride.

Since each Ca+2 surrounded by 8 F ions and

each F is surrounded by 4 Ca+2 ions.

Thus, the structure has 8 : 4 coordination.

Now think about structure of ZnS.

ZnS The arrangement as possessed by ZnS is called cubic close packing (ccp) in which S2 ions are present at the corners as well as at the centre of each face of the cube.

Each Zn2+ ion is surrounded tetrahedrally by four S2 ions and each S2 ion is surrounded tetrahedrally by four Zn2+ ions. Thus, this structure has 4 : 4 coordination.

Ans: (b) and (c)


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