Structures of Ionic Solids MCQ - Practice Questions & Answers

Quick Facts

Structure of Some Ionic Solids

NaCl (Rock Salt)

In it CI^- ions have CCP structure that is, FCC and Na⁺ ions occupies octahedral holes (voids) here.
Both Na⁺ and CI^- have cooordination number six. The number of formula units per unit cell is four. Example, halides of alkali metals (except CsX) like LiX, NaX, KX, other halides like AgCl, AgBr, NH₄Cl oxides like TiO, FeO, NiO.
r_c+ r_a= a/2 and r_Na₊/r_cl- must be 0.414 however, it is 0.525.

CsCl

Here Cl^-ions are at the comers of a cube and Cs⁺ ions are in the cubic void (centred position) i.e. BCC like structure.
Here coordination number of both Cs⁺and Cl^- is 8 and here the number of formula units per unit cell is 1. r_c + r_a= √3a/2.
Here r_Cs_⁺/r_Cl_⁺should be 0.732 but it is 0.93, e.g. CsX, TiBr, NH₄Cl, NH₄Br

ZnS (Zinc-Blende)

Here S^2- occupies CCP or FCC arrangement while Zn²⁺ ions occupy alternate tetrahedral voids.
Only half of the total voids are occupied.
Here coordination number of both Zn²⁺ and S^2- is four and here the number of formula units per unit cell is four. For example, ZnS, CuBr, Cul, Agl, etc.
r_c + r_a = √3a/4

CaF₂ (Fluorite Structure)

Here Ca²⁺ occupies CCP and F^- ions occupy all the tetrahedral voids.
Here coordination number of Ca²⁺ is 8 while for F^- it is four and the effective number of Ca²⁺ and F^-ions in a unit cell are 4 and 8 respectively, here the number of formula units per unit cell is four.
r_c + r_a = √3a/4

Na₂O (Anti-Fluorite Structure)

Here negative ions (O^2-) occupy CCP while cations Na⁺occupy all the tetrahedral voids.
Here coordination number of Na⁺ is four while for O^2- it is eight, here the number of formula units per unit cell is four and the effective number of Li⁺and O^2- ions in a unit cell are 8 and 4 respectively.
r_c + r_a = √3a/4

Normal Spinel Structure (XY₂O₄)

Spinel is MgAl₂O₄, and spinel, in general, have the formula AB₂O₄. Here A is a divalent cation (Mg²⁺, Ca²⁺) and B is trivalent cation Al³⁺.
Here Mg²⁺occupy 1/8th of tetrahedral voids while oxide ions 1/ 8 of Al³⁺ occupy 1/2 of the octahedral voids.
As magnetic materials, these are used in telephones, memory loops of computers.

Inverse Spinel Structure (Fe₃O₄, Magnetite)

In Fe₃O₄, Fe³⁺and Fe²⁺are present in 2:1 ratio.
Here oxides ions are in CCP while Fe²⁺ions occupy 1 octahedral void while Fe³⁺ occupy 1 octahedral and 1 tetrahedral voids.
Here O^2- ions form the FCC unit cell.
Here the formula ratio of
Fe²⁺ : Fe³⁺ : O^2-= 1 : 2 : 4
MgFe₂O₄, Pb₃O₄, Mn₃O₄ have also this type of structure.

Effect of T and P on Crystal Structure

On increasing the temperature of the CsCl structure coordination number decreases from 8:8 to 6:6.
$\mathrm{CsCl} \stackrel{\Delta}{\longrightarrow} \mathrm{NaCl\: Structure}$
8:8 6:6
On subjecting the NaCl structure to high pressure it will increase coordination number from 6:6 to 8:8.
$\mathrm{NaCl} \stackrel{\text { highly pressurized }}{\longrightarrow}\mathrm{CsCl\: Structure}$
6:6 8:8