Define coordination number of a metal ion in an ionic crystal

Explanation

**Step-by-Step Solution:**

1. **Understanding Coordination Number**: The coordination number is a term used in chemistry to describe the number of ions or molecules that are directly surrounding a central atom in a complex or crystal structure.

2. **Definition in Context**: In the context of an ionic crystal, the coordination number specifically refers to the number of negatively charged ions (anions) that are in close proximity to a positively charged metal ion (cation).

3. **Identifying Surrounding Ions**: To determine the coordination number, one must identify the anions that are nearest to the metal ion in the crystal lattice structure.

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