What is the co-ordination number of sodium in `Na_(2)O` ?

Answer

To determine the coordination number of sodium (Na) in sodium oxide (Na₂O), we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Structure Type**:
Sodium oxide (Na₂O) adopts an anti-fluorite structure. In this structure, the oxide ions (O²⁻) are larger than the sodium ions (Na⁺).

2. **Arrangement of Ions**:
In the anti-fluorite structure, the oxide ions occupy the cubic close-packed (CCP) lattice positions, while the sodium ions occupy the tetrahedral voids within this lattice.

3. **Determine the Number of Ions**:
In a cubic close-packed structure, the number of atoms (or ions) per unit cell, denoted as Z, is 4. Since the oxide ions are in the CCP, there are 4 O²⁻ ions per unit cell.

4. **Calculate Tetrahedral Voids**:
The number of tetrahedral voids in a cubic close-packed structure is twice the number of atoms in the lattice. Therefore, for 4 O²⁻ ions, there are 8 tetrahedral voids available.

5. **Distribution of Sodium Ions**:
Since there are 4 O²⁻ ions in the unit cell, and each O²⁻ ion can coordinate with 4 Na⁺ ions, we can conclude that the coordination number of Na⁺ is determined by the number of tetrahedral voids it occupies.

6. **Coordination Number Calculation**:
The coordination number of Na⁺ in Na₂O is 4, as each Na⁺ ion is surrounded by 4 O²⁻ ions.

### Final Answer:
The coordination number of sodium (Na) in Na₂O is **4**.

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