Ionic Compound Of The Type AB2

(i) The Ca⁺² ions are arranged in ccp arrangement, i.e. these ions occupy all the corners and the centres of each face of the cube

(ii) The F– ions occupy all the tetrahedral holes.

(iii) Since there are two tetrahedral holes for each Ca⁺² ion and F− ions occupy all the tetrahedral holes, there will be two F− ions for each Ca+2 ions, thus the stoichiometry of the compound is 1:2.

(iv) Each Ca+2 ion is surrounded by 8F- ions and each F− ions is surrounded by 4Ca⁺² ions. The Coordination number of Ca⁺² ion is eight and that of F− ion is four, this is called 8:4 Coordination.

(i) In the crystal structure of Li₂O, the O^-2 ions constitute a cubic close packed lattice (fcc structure) and the Li+ ions occupy all the tetrahedral voids

(ii) Each oxide ion, O^-2 ion is in contact with 8 Li+ ions and each Li+ ions having contact with 4 oxide ion. Therefore, Li₂O has 4:8 coordination

Examples – Na₂O, K₂O, K₂S, Na₂S, Rb₂O, Rb₂S

Crystal structure

Brief description

Coordination number

No. of atoms per unit cell

Examples

1.

Rock salt (NaCl – type)

Cl- ions in ccp Na+ ions occupy all octahedral voids

Na+ = 6

Cl- = 6

4

Li, Na, KI, and Rb halides

NH4Cl, NH₄Br, NH₄I, AgF, AgCl, AgBr, MgO, CaO, TiO, FeO, NiO

2.

Zinc blende (ZnS – types)

S-2 ions in ccp Zn⁺² ions occupy alternate tetrahedral voids

Zn⁺² = 4

S-2 = 4

4

ZnS, BeS, CuCl, CuBr, CuI, AgI, HgS

3.

Wurtzite (ZnS – type)

S-2 ions in hcp Zn⁺² ion occupy alternate tetrahedral voids

Zn⁺² = 4

S-2 = 4

4

ZnS, ZnO, CdS, BeO

4.

Caesium chloride (CsCl type)

Cl- ions in bcc Cs+ ions in the body of cube

Cs⁺² = 8

Cl- = 4

1

CsCl, CsBr, CsI, CsCN,CaS

5.

Fluorite (CaF₂ type)

Ca+2 ions in ccp, F- ions occupy all tetrahedral voids

Ca⁺² = 8

F- = 4

4

CaF₂, SrF₂, BaF₂, BaCl₂, SrCl₂, CdF₂, HgF₂

6.

Anti fluorite (Li₂O – type)

O-2 ions in ccp, Li+ ions occupy all tetrahedral sites

Li+ = 4

O-2 = 8

4

K₂O, Na₂O, K₂S, Na₂S