Question

The correct statement in the following is
(A) The ionic crystal of $AgBr$ has only a Schottky defect
(B) The unit cell having crystal parameters, $a=b\ne c$ , $\alpha =\beta ={{90}^{o}}$ , $\gamma ={{120}^{o}}$ is hexagonal
(C) In ionic compounds having Frenkel defect the ratio $\frac{{{\gamma }_{+}}}{{{\gamma }_{-}}}$ is high
(D) The coordination number of $N{{a}^{+}}$ ion in $NaCl$ is 4

Answer 1

Hint: The Schottky defect and frenkel defect are two types of point defects. These defects decrease the stability of the crystal. The smallest repeating unit in a crystal lattice is called a unit cell. The number of atoms, ions, or molecules that a central atom or ion holds as its closest neighbours in a complex is known as the coordination number.

Complete step by step solution:
(A) The ionic crystal of $AgBr$ has only a Schottky defect.
Schottky defect, which is simply a vacancy defect, happens when the lattice structure lacks an equal number of cations and anions. It is quite common for both the anion and the cation to be the same size in ionic compounds with high coordination numbers. $AgBr$ has not only a Schottky defect but a Frenkel defect also. This statement is wrong.

(B) The unit cell having crystal parameters, $a=b\ne c$ , $\alpha =\beta ={{90}^{o}}$ , $\gamma ={{120}^{o}}$ is hexagonal.
This statement is correct.

(C) In ionic compounds having a Frenkel defect, the ratio $\frac{{{\gamma }_{+}}}{{{\gamma }_{-}}}$ is high.
 The Frenkel defect occurs in ionic solids where the sizes of the cation and anion differ. The Frenkel defect exists in the ionic compounds, but the ratio $\frac{{{\gamma }_{+}}}{{{\gamma }_{-}}}$ is low. Hence, the statement is wrong.

(D) The coordination number of $N{{a}^{+}}$ ion in $NaCl$ is 4.
This statement is wrong as the coordination number of $N{{a}^{+}}$ ion in $NaCl$ is 6.
Correct Option: (B) The unit cell having crystal parameters, $a=b\ne c$ , $\alpha =\beta ={{90}^{o}}$ , $\gamma ={{120}^{o}}$ is hexagonal

Note: A unit cell is the smallest collection of symmetrically aligned atoms that can be repeated in an array to form the whole crystal. Some crystals having hexagonal unit cells are zinc oxide and beryllium oxide.