`NiO` adopts a rock - salt structure , The coordination number of the `Ni^(2+)` ion is

Answer

To determine the coordination number of the `Ni^(2+)` ion in nickel oxide (`NiO`), which adopts a rock salt structure, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Rock Salt Structure**:
- The rock salt structure is similar to that of sodium chloride (NaCl). In this structure, the cations and anions are arranged in a face-centered cubic (FCC) lattice.

2. **Arrangement of Ions**:
- In the rock salt structure, the anions (in this case, `O^(2-)`) occupy the face-centered positions of the cubic unit cell, while the cations (`Ni^(2+)`) occupy the octahedral holes.

3. **Coordination Number Definition**:
- The coordination number of an ion in a crystal lattice is defined as the number of nearest neighboring ions surrounding it.

4. **Identifying Nearest Neighbors**:
- In the rock salt structure, each `Ni^(2+)` ion is surrounded by six `O^(2-)` ions. This is because in an octahedral arrangement, there are six points of contact.

5. **Conclusion**:
- Therefore, the coordination number of the `Ni^(2+)` ion in `NiO` is 6.

### Final Answer:
The coordination number of the `Ni^(2+)` ion in nickel oxide (`NiO`) is **6**.

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