Crystal structures refer to the orderly arrangement of atoms, ions, or molecules in a solid. These structures are characterized by repeating patterns, which can be described using a unit cell, the smallest repeating unit that reflects the overall symmetry and properties of the crystal. Common types of crystal systems include cubic, tetragonal, and hexagonal, each defined by specific geometric parameters. The arrangement of particles in a crystal affects its physical properties, such as melting point, hardness, and conductivity. For example, the diamond crystal structure contributes to its exceptional hardness, while the cubic structure of table salt (sodium chloride) influences its solubility in water. Understanding crystal structures is essential in fields like materials science and mineralogy.