Question

Ionic Compounds of AB₂ Type.

Answer 1

Ionic Compounds of AB₂ Type-

Fluorite structure (crystal structure of CaF₂): The calcium fluoride crystal is composed of calcium ions (Ca²⁺) and fluoride ions (F^-). Their respective arrangement in the unit cell or crystal lattice is shown in Figure.

(i) As shown in Figure, the Ca²⁺ ions are present at the face centred cubic lattice and the fluoride ions (F^-) occupy all the tetrahedral voids.

(ii) The radius ratio, calculated from the ionic radii of Ca²⁺ ion (99 pm) and F^- ion (136 pm), is 0.73. This shows that the coordination number of calcium ion is eight, i.e. each calcium cation is surrounded by eight fluoride anions in a body centred cubic arrangement. Each fluoride ion is in contact with four calcium ions. Thus CaF₂ has 8 : 4 co-ordination.

(iii) Each unit cell of CaF₂ crystal is observed to have 4Ca²⁺ ions and 8F^- ions

Number of Ca²⁺ ions = (8 ions at the corners × \(\frac 18\)) + (6 ions at the face centre × \(\frac 18\))

= 1 + 3

= 4

Number of F^- ions (8 ions within the body x 1) = 8

Therefore, each unit cell has 4 CaF₂ molecules.

Solids forming fluorite type of structure are CaF₂, SrF₂, BaF₂, BaCl₂, CdF₂, HgF₂, PbF₂, CuF₂, SrCl₂ etc.