When a crystal having rock salt type geometry is heated in the presence of it's metal vapour then defect in it will be

Options

A. Stoichiometric defect
B. Metal excess defect
C. Anion excess defect
D. Frenkel defect

Answer

To solve the question regarding the defect that occurs when a crystal with rock salt type geometry is heated in the presence of its metal vapor, we can follow these steps:

### Step 1: Understand Rock Salt Structure
Rock salt (NaCl) type geometry refers to a face-centered cubic (FCC) lattice structure where sodium (Na+) and chloride (Cl-) ions are arranged alternately. Each ion occupies specific lattice points, and the structure is held together by ionic bonds.

**Hint:** Remember the arrangement of ions in a rock salt structure and how they interact with each other.

### Step 2: Identify the Effect of Heating in Metal Vapor
When a rock salt crystal is heated in the presence of its metal vapor (for example, Na vapor for NaCl), the metal atoms can deposit onto the crystal surface. This can lead to changes in the ionic balance within the crystal.

**Hint:** Consider how the introduction of metal vapor might affect the existing ions in the crystal.

### Step 3: Consider Electrical Neutrality
To maintain electrical neutrality in the crystal, the introduction of additional metal ions (from the vapor) must be balanced by the removal of some anions (negatively charged ions). This results in the creation of vacancies, specifically anion vacancies.

**Hint:** Think about how the charge balance must be maintained when additional positive charges are introduced.

### Step 4: Define the Type of Defect
The defect created due to the removal of anions to maintain electrical neutrality in the presence of excess metal ions is known as a "metal excess defect." This defect arises specifically due to anion vacancies.

**Hint:** Recall the terminology used in solid-state chemistry to describe defects related to ionic compounds.

### Step 5: Conclusion
Thus, when a crystal with rock salt type geometry is heated in the presence of its metal vapor, the defect that occurs is a metal excess defect, characterized by anion vacancies.

**Final Answer:** The defect in the crystal will be a **metal excess defect** due to anion vacancies.

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