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Question

Match the following:

CompoundsCoordination(A:B)(1)ZnS(i)8:8(2)CsCl(ii)6:6(3)NaCl(iii)4:4(4)Na2O(iv)8:4(5)CaF2(v)4:8


A

1 – (iii), 2 – (i), 3 – (ii), 4 – (v), 5 – (iv)

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B

1 – (ii), 2 – (i), 3 – (iii), 4 – (v), 5 – (iv)

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C

1 – (iii), 2 – (i), 3 – (ii), 4 – (iv), 5 – (v)

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D

1 – (ii), 2 – (iii), 3 – (i), 4 – (v), 5 – (iv)

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Solution

The correct option is A

1 – (iii), 2 – (i), 3 – (ii), 4 – (v), 5 – (iv)


Let’s try to recall all the given structures.

NaCl It has a face-centered cubic (fcc) arrangement (also called cubic close packing, i.e., ccp) in which Cl ions occupy the corners and face centers of the cube which Na ions are present at the body centre and edge centers.

NaCl structure

Each Na ion is surrounded by six Cl ions and each Cl ion is surrounded by six Na ions. In other words, the Na ions as well as Cl ions have coordination number of 6, i.e., this structure has 6 : 6 coordination (i.e., Cl ions being larger in size are present in the close packing while Na ions are present in all the octahedral voids.

CsCl

It has body-centered cubic (bcc) arrangement.

Each Cs ion is surrounded by eight Cl ions and each Cl ion is surrounded by eight Cs ions, i.e., this structure has 8: 8 coordination.

A unit cell of CsCl consists of only one unit of CsCl, i.e., one Cs ion and one Cl ion (because each ion present at the corner is shared by eight unit cells) [The ratio rCsrCl = 0.93 which is greater than expected for exact fitting (0.732)].

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.

CaF2 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.

Na2O It is an antiflourite type structure so the cations and anions’ positions will be reversed as they are in fluorite type (CaF2) type structure.

Coordination number of A2B -type structure:

Each O2 ions is surrounded by eight Na ions.

Each Na ion is surrounded by four O2.

Thus, the structure has 4 : 8 coordination.


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