Which of the following statements is correct in the rock-salt structure of ionic compounds?

Options

A. Co-ordination number of cation is four and anion is six
B. Co-ordination number of cation is six and anion is four
C. Co-ordination number of each cation and anion in four
D. Co-ordination number of each cation and anion in six

Answer

To solve the question regarding the correct statement in the rock-salt structure of ionic compounds, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Rock Salt Structure**:
- The rock salt structure is commonly represented by sodium chloride (NaCl). In this structure, sodium ions (Na⁺) and chloride ions (Cl⁻) are arranged in a specific way.

2. **Draw the Unit Cell**:
- Visualize the unit cell of the rock salt structure. In the cubic unit cell, chloride ions (Cl⁻) occupy the corners of the cube, while sodium ions (Na⁺) are located in the octahedral voids.

3. **Identify the Coordination Number**:
- The coordination number of an ion is defined as the number of nearest neighboring ions surrounding it.
- For the chloride ion (Cl⁻), it is surrounded by 6 sodium ions (Na⁺) located at the vertices of the octahedron formed around it.
- Similarly, for the sodium ion (Na⁺), it is also surrounded by 6 chloride ions (Cl⁻).

4. **Conclusion**:
- Since both the sodium and chloride ions have a coordination number of 6, the correct statement regarding the rock salt structure is that the coordination number of both the cation (Na⁺) and the anion (Cl⁻) is 6.

### Final Answer:
The correct statement in the rock-salt structure of ionic compounds is: "The coordination number of cation and anion is 6."

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