Sodium chloride crystallises in a face centred cubic lattice in which each

Options

A. Sodium ion is tetrahedrally surrounded by 4 chloride ions and each chloride ion is tetrahedrally surrounded by 4 sodium ions
B. sodium ion is tetrahedrally by 4 chloride ions and each chloride ion is octahedrally surrounded by 6 sodium ions
C. sodium ion is octahedrally surrounded by 6 chloride ions and each chloride ion is surrouneded by 4 sodium ions
D. sodium ion is octahedrally surrouned by 6 chloride ions and each chloride ion is octahedrally surrounded by 6 sodium ions

Answer

To solve the question regarding the crystallization of sodium chloride (NaCl) in a face-centered cubic (FCC) lattice, we will analyze the structure and the coordination of ions in the lattice.

### Step-by-Step Solution:

1. **Understanding the FCC Structure**:
- Sodium chloride crystallizes in a face-centered cubic lattice. In this type of lattice, the unit cell has atoms at each corner and at the center of each face of the cube.

2. **Identifying the Ions**:
- In NaCl, sodium ions (Na⁺) and chloride ions (Cl⁻) are present in equal numbers. The Na⁺ ions occupy specific positions in the lattice.

3. **Position of Sodium Ions**:
- The sodium ions are located at the corners and the centers of the faces of the cubic unit cell. This arrangement allows each sodium ion to interact with neighboring chloride ions.

4. **Coordination of Sodium Ions**:
- Each sodium ion is surrounded by six chloride ions. This arrangement forms an octahedral geometry, where the sodium ion is at the center and the chloride ions are at the vertices of the octahedron.

5. **Position of Chloride Ions**:
- Conversely, the chloride ions occupy the octahedral voids created by the arrangement of sodium ions. Each chloride ion is also surrounded by six sodium ions in an octahedral configuration.

6. **Conclusion**:
- Therefore, in the NaCl structure, each sodium ion is octahedrally coordinated by six chloride ions, and each chloride ion is also octahedrally coordinated by six sodium ions. This leads us to the conclusion that the correct answer is option four: "Sodium ion is octahedral surrounded by six chloride ions and each chloride ion is octahedral surrounded by six sodium ions."

### Final Answer:
The correct option is **Option 4**: Sodium ion is octahedral surrounded by six chloride ions and each chloride ion is octahedral surrounded by six sodium ions.