What is a crystal structure defined by?

A crystal structure is defined by specific geometric patterns of structural units. In a crystalline solid, the atoms, ions, or molecules are arranged in an orderly and repeating three-dimensional pattern. This regular arrangement leads to the formation of distinct geometric shapes, which is a hallmark characteristic of crystalline solids. The repeating unit in this arrangement is known as the unit cell, which defines the symmetry and overall shape of the crystal.

In contrast, irregular configurations of atoms would represent an amorphous solid rather than a crystalline one, where there is no long-range order. Random arrangements of molecules imply a lack of organization, which is also characteristic of non-crystalline materials. Lastly, liquid-like structures at low temperatures suggest a state of matter that does not maintain a fixed geometric pattern; instead, these structures are typically found in liquids or gels, not in true crystalline forms. Hence, the definition of a crystal structure relies fundamentally on its ordered, repeating geometric pattern.