What does the coordination number refer to in crystal systems?

The coordination number is an important concept in crystallography and refers specifically to the number of nearest neighbors surrounding a central atom in a crystal lattice. In simpler terms, it indicates how many other atoms, ions, or molecules are directly touching a particular atom within the crystal structure.

In a crystal system, the coordination number varies depending on the arrangement of the atoms and the geometry of the crystal. For instance, in a face-centered cubic structure, each atom has a coordination number of 12, as it is surrounded by 12 nearest neighbors. Understanding the coordination number helps in analyzing properties such as stability, packing efficiency, and types of bonding between atoms in different crystal systems.

The other options do not accurately define the coordination number. The number of atoms in the cell refers to a broader count of all atoms within the unit cell rather than the specific interactions with nearest neighbors. The volume of the unit cell is a physical measure of space occupied and does not relate directly to the connectivity of atoms. Lastly, the distance between lattice points pertains to the geometry of the crystal structure but does not convey information about how many atoms are adjacent to a given atom. Thus, option C correctly identifies the specific relational aspect that defines coordination number in crystal systems.