A compound `AB_(2)` possesses the `CaF_(2)` type crystal structure. The co-ordination number of `A^(2+)` and `B^(–)` ions is the crystal will be:

Options

A. 8, 4
B. 4, 8
C. 12, 6
D. 6, 12

Answer

To solve the question regarding the coordination numbers of ions in a compound \( AB_2 \) with a \( CaF_2 \) type crystal structure, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Structure**:
The compound \( AB_2 \) is stated to have a \( CaF_2 \) type crystal structure. In this structure, \( A^{2+} \) represents the cation (similar to \( Ca^{2+} \)), and \( B^{-} \) represents the anion (similar to \( F^{-} \)).

**Hint**: Recognize that the structure type gives insight into how the ions are arranged in the crystal lattice.

2. **Understand Coordination Numbers**:
The coordination number of an ion is defined as the number of nearest neighboring ions surrounding it. For \( CaF_2 \):
- Each \( Ca^{2+} \) ion is surrounded by 8 \( F^{-} \) ions.
- Each \( F^{-} \) ion is surrounded by 4 \( Ca^{2+} \) ions.

**Hint**: Recall that the coordination number reflects how many ions are directly adjacent to a given ion.

3. **Determine Coordination Number for \( A^{2+} \)**:
Since \( A^{2+} \) is analogous to \( Ca^{2+} \) in the \( CaF_2 \) structure, the coordination number for \( A^{2+} \) will also be 8.

**Hint**: Use the analogy between \( A^{2+} \) and \( Ca^{2+} \) to deduce the coordination number.

4. **Determine Coordination Number for \( B^{-} \)**:
Similarly, since \( B^{-} \) is analogous to \( F^{-} \), the coordination number for \( B^{-} \) will be 4.

**Hint**: Again, leverage the analogy between \( B^{-} \) and \( F^{-} \) to find the coordination number.

5. **Summarize the Results**:
- The coordination number of \( A^{2+} \) is 8.
- The coordination number of \( B^{-} \) is 4.

### Final Answer:
- The coordination number of \( A^{2+} \) is **8**.
- The coordination number of \( B^{-} \) is **4**.