The number of ions of opposite charge surrounding each ion in a crystal is called the coordination number. It's a fundamental concept in crystallography and describes the geometry of the crystal lattice.

Here's a breakdown:

* Coordination Number: This number represents the number of nearest neighbors of opposite charge that surround a particular ion in a crystal lattice.

* Crystal Lattice: This is a three-dimensional repeating pattern of atoms or ions that forms the structure of a crystalline solid.

* Opposite Charge: The ions in a crystal lattice are held together by electrostatic forces between ions with opposite charges (positive and negative).

Factors Affecting Coordination Number:

* Ionic Radii: The relative sizes of the ions involved. Smaller ions can fit more closely around a larger ion, leading to a higher coordination number.

* Charge Densities: The ratio of charge to volume of the ions. Higher charge densities can lead to stronger electrostatic interactions and thus higher coordination numbers.

Examples:

* Sodium Chloride (NaCl): In the NaCl crystal structure, each sodium ion (Na+) is surrounded by six chloride ions (Cl-), and each chloride ion is surrounded by six sodium ions. The coordination number for both Na+ and Cl- is 6.

* Cesium Chloride (CsCl): In the CsCl structure, each cesium ion (Cs+) is surrounded by eight chloride ions (Cl-), and each chloride ion is surrounded by eight cesium ions. The coordination number for both Cs+ and Cl- is 8.

Important Note: The coordination number is a specific value for each crystal structure and can vary depending on the type of ions involved and the crystal structure itself.