What is the coordination number of
(i) sodium in sodium oxide `(Na_2O)`?
(ii) oxide ion in sodium oxide `(Na_2O)`?
(iii) calcium in calcium fluoride `(CaF_2)`?
(iv) zinc in zinc blende (ZnS)?

Explanation

To solve the question regarding the coordination numbers in the given compounds, we will analyze each part step by step.

### Step 1: Coordination Number of Sodium in Sodium Oxide (Na₂O)

1. **Identify the ions**: In sodium oxide (Na₂O), we have sodium ions (Na⁺) and oxide ions (O²⁻).
2. **Determine the arrangement**: The oxide ion (O²⁻) has a cubic close-packed (CCP) arrangement.
3. **Identify the site occupied by sodium**: Sodium ions occupy tetrahedral sites in this arrangement.
4. **Calculate the coordination number**: The coordination number for an ion occupying tetrahedral sites is 4.
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