Crystal structure refers to the orderly arrangement of atoms, ions, or molecules in a crystalline material. This arrangement forms a repeating pattern in three-dimensional space, which is essential for determining the material's properties, such as strength, conductivity, and optical characteristics. Common examples of crystal structures include sodium chloride (table salt) and diamond. The basic unit of a crystal structure is called a unit cell, which is the smallest repeating unit that reflects the symmetry and arrangement of the entire crystal. Different types of crystal systems, such as cubic, tetragonal, and hexagonal, describe the various shapes and dimensions of these unit cells, influencing how the material interacts with light and other substances.