Question

Assertion: In any ionic solid [MX] with Schottky defects, the numbers of positive and negative ions are the same.
Reason: Equal number of cation and anion vacancies are present.
(A) Both assertion and reason are true and reason is the correct explanation of assertion
(B) Both assertion and reason are true and reason is not the correct explanation of assertion
(C) Assertion is true but reason is false
(D) Both assertion and reason are false

Answer 1

To answer this question, you must recall the Schottky defect in a crystal. A defect or imperfection in a crystal is caused by the departure of any particle from the perfectly ordered arrangement of atoms/ ions in a crystal. Schottky defect is a stoichiometric defect.

Complete step by step solution
Stoichiometric defects are those defects in a crystal in which the ratio of the anions and the cations remain the same as is represented by the molecular formula. Stoichiometric defects are further classified into Schottky defects, Frenkel defects and impurity defects.
Schottky defects are the defects that arise when in ionic crystals, equal numbers of anions and cations are missing from the lattice sites so that the electrical neutrality is maintained. It is generally shown by highly ionic compounds that have high coordination numbers and cations and anions of similar size. Common examples are sodium chloride, potassium chloride, potassium bromide and cesium chloride.
Schottky defect in a crystal tends to decrease the density of a crystal since the number of ions in a fixed volume decreases. Also as holes are created in the crystal lattice, the stability and lattice energy of the crystal decreases.
Thus, we can say from the above statements that both the assertion and reason statements are true and the reason gives the correct explanation of the assertion.
The correct answer is A.

Note
Defects in ionic crystals are broadly of two types, namely electronic imperfections and atomic or pint imperfections. Atomic or point imperfections are defects which arise due to the irregularity in the arrangement of atoms or ions. They can be classified into stoichiometric defects, non- stoichiometric defects and impurity defects. For this question, we have to focus only on the stoichiometric defects.