Define coordination number of a metal ion an ionic crystal.

Explanation

**Step-by-Step Solution:**

1. **Understanding Coordination Number**: The coordination number of a metal ion in an ionic crystal refers to the number of atoms, ions, or molecules that are directly surrounding and bonded to that metal ion.

2. **Ionic Crystal Structure**: In an ionic crystal, metal ions are surrounded by non-metal ions (or vice versa) in a specific geometric arrangement. This arrangement is determined by the size and charge of the ions involved.

3. **Defining the Coordination Number**: The coordination number is essentially a count of how many nearest neighbors (atoms or ions) are in contact with the central metal ion.

...