Which one of the following statements is incorrect about rock salt type ?

Options

A. It has fcc arrangement of `CI^(-)`
B. `Na^(+) and Cl^(-)` ions have a co-ordination number of `6:6`
C. A unit cell of NaCl consists of four Nacil units
D. All halides of alkali metals have rock-salt type structure

Answer

To determine which statement is incorrect about the rock salt type structure, we will analyze each option provided in the question.

### Step-by-Step Solution:

1. **Understanding Rock Salt Structure**:
- Rock salt, also known as sodium chloride (NaCl), has a face-centered cubic (FCC) arrangement. In this structure, Na⁺ and Cl⁻ ions alternate in the lattice.

**Hint**: Recall that the rock salt structure is characterized by an FCC arrangement.

2. **Analyzing Option A**:
- **Statement**: "It has a FCC arrangement of Cl⁻."
- This statement is correct because in the rock salt structure, Cl⁻ ions indeed occupy the face-centered positions of the FCC lattice.

**Hint**: Check the arrangement of ions in the FCC structure to confirm this.

3. **Analyzing Option B**:
- **Statement**: "Na⁺ and Cl⁻ ions have coordination number of 6."
- This statement is also correct. In the NaCl structure, each Na⁺ ion is surrounded by six Cl⁻ ions and vice versa, giving a coordination number of 6 for both ions.

**Hint**: Remember that coordination number refers to the number of nearest neighbors surrounding an ion.

4. **Analyzing Option C**:
- **Statement**: "Units of NaCl consist of four NaCl units."
- This statement is correct as well. In the unit cell of NaCl, there are effectively four NaCl formula units per unit cell.

**Hint**: Consider how many Na⁺ and Cl⁻ ions are present in the unit cell to verify this.

5. **Analyzing Option D**:
- **Statement**: "All halides of alkali metals have rock salt type structure."
- This statement is incorrect. While NaCl has a rock salt structure, not all alkali metal halides do. For example, CsBr, CsCl, and CsI have different structures (like cesium chloride structure) and do not follow the rock salt type.

**Hint**: Think about the structures of different alkali metal halides and their coordination.

### Conclusion:
The incorrect statement about rock salt type is **Option D**: "All halides of alkali metals have rock salt type structure."

### Final Answer:
**Option D** is the correct answer to the question.