7.0 Classification of Ionic Structures

  Solid State

Simple ionic compounds are of the type $AB$ or $AB_2$ where $A$ and $B$ represent the positively and negatively charged ions respectively. (In any solid of the type $A_xB_y$, the ratio of co-ordination number of $A$ to $B$ would be $y:x$.

Structures of Type $AB$:

Ionic compounds of the type $AB$ means compounds having the positively and negatively charged ions in the ratio $1:1$. These compounds can have following three type of structures.

$1.$ Rock salt $(NaCl)$ type

$2.$ Zinc blende $(ZnS)$ type

$3.$ Cesium chloride $(CsCl)$ type

$1.$ Rock Salt $(NaCl)$ type Structure:

$Cl^–$ is forming a FCC unit cell in which $Na^+$ is in the octahedral voids.

The co-ordination number of $Na^+$ is $6$ and that of $Cl^–$ would also be $6$. Moreover, there are $4$ $Na^+$ ions and $4$ $Cl^–$ ions per unit cell.

The formula is $Na_4Cl_4$ i.e. $NaCl$.

Other examples for this type of structure are all halides of alkali metals except $CsCl$ and all oxides of alkaline earth metals except $BeO$.

$2.$ Zinc Blende Structure $(ZnS)$:

Sulphite ions are face centred and zinc is present in alternate tetrahedral voids.

Formula is $Zn_4S_4$, i.e. $ZnS$. Coordination number of $Zn$ is $4$ and that of sulphide is $4$. Other substance that exists in this kind of a structure is $BeO$.

Zinc blende has diamond like structure in which cation and have similar surroundings anions.

$3.$ Cesium Chloride $(CsCl)$ type structure:

$CsCl$ has body-centred cubic (bcc) arrangement. Each $Cs^+$ ion is surrounded by $8$ $Cl^–$ ions and each $Cl^–$ ion is surrounded by $8$ $Cs^+$ ions i.e. this structure has $8:8$ co-ordination.

A unit cell of $CsCl$ consists of only one unit of $CsCl$ i.e. One $Cs^+$ ion and one $Cl^–$ ion. Few examples of compounds having $CsCl$ structure are $CsBr$, $CsI$, $CsCN$, $TlCl$, $TlBr$, $TlI$ and $TlCN$.

Structure of Ionic Compounds of the Type $AB_2$:

These are the ionic compounds having cations and anions in the ratio $1:2$. Most of these compounds have calcium fluorite $(CaF_2)$ type structure. These compounds have cubic close packing (CCP) arrangement in which $Ca^{2+}$ ions are present at the corners and the centre of each face of the cube.

Each $Ca^{2+}$ ion is surrounded by $8F^–$ ions i.e. it has a co-ordination number of $8$ whereas each $F^–$ ion is surrounded by $4$ $Ca^{2+}$ ions i.e. has a co-ordination number of $4$.

Thus this structure has $8:4$ co-ordination. Few examples of such compounds having $CaF_2$ structure are $BaF_2$, $BaCl_2$, $SrF_2$, $SrCl_2$, $CdF_2$, $PbF_2$ and $ThO_2$.

In $Na_2O$ each oxide ion is co-ordinated to $8$ $Na^+$ ions and each $Na^+$ ion to $4$ oxide ions. Hence, it has $4:8$ co-ordination. This is called anti-fluorite structure. Others examples being $Cl_2O$, $K_2O$, $Li_2O$, $K_2S$, $Na_2S$ etc.