Any deviation from the perfectly ordered arrangement constitutes a defect or imperfection. The compounds in which the number of irregularity are present in the arrangement of atoms or ions are called atomic imperfections or point defects. These may be stochiometric or non - stoichlometric. Stoichiometric defects are further classified into Schottky defects in which number of positive and negative ions are missing in equal ratio and Frenkel defects in which an ion leaves its correct lattice sites and occupies on interstitial site. Schottky defect is more common in ionic compounds with high co-ordination number and where the size of positive and negative ions are almost equal, while in Frenkel defects are common in ionic compounds, which have low co-ordination number and in which there is large difference in size between positive and negative ions.
In a crystal some ions are missing from normal sites. This is an example of

Options

A. F-centres
B. Interstitial defect
C. Frenkel defect
D. Schottky defect

Answer

To solve the question regarding the type of defect in a crystal where some ions are missing from their normal sites, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Concept of Defects in Crystals**:
- In solid-state chemistry, defects or imperfections refer to deviations from the perfect arrangement of atoms or ions in a crystal lattice. These defects can affect the properties of the material.

2. **Types of Defects**:
- Defects in crystals are categorized into two main types: **stoichiometric defects** and **non-stoichiometric defects**.
- Stoichiometric defects can be further classified into **Schottky defects** and **Frenkel defects**.

3. **Schottky Defects**:
- A Schottky defect occurs when an equal number of positive and negative ions are missing from their lattice sites. This results in vacancies in the crystal structure.
- Schottky defects are more common in ionic compounds that have a high coordination number and where the sizes of the cations and anions are similar.

4. **Frenkel Defects**:
- A Frenkel defect occurs when an ion (usually a cation) leaves its lattice site and occupies an interstitial site within the crystal. This creates a vacancy at the original lattice position and an interstitial defect.

5. **Analyzing the Given Question**:
- The question states that "some ions are missing from normal sites." This indicates that there are vacancies in the lattice.
- Since the question does not mention any ions migrating to interstitial sites, we can conclude that this scenario aligns with the definition of a Schottky defect.

6. **Conclusion**:
- Therefore, the correct answer to the question is that the missing ions from normal sites in a crystal is an example of a **Schottky defect**.

### Final Answer:
The correct answer is **Schottky defect**.