Molecular structure of crystal.

THE scientific definition of a "crystal" is based on the microscopic arrangement of atoms inside it, called the crystal structure. A crystal is a solid where the atoms form a periodic arrangement.

A crystal structure (an arrangement of atoms in a crystal) is characterised by its unit cell, a small imaginary box containing one or more atoms in a specific spatial arrangement. The unit cells are stacked in three-dimensional space to form the crystal.

The symmetry of a crystal is constrained by the requirement that the unit cells stack perfectly with no gaps. There are 219 possible crystal symmetries, called crystallographic space groups. These are grouped into 7crystal systems, such as cubic crystal system (where the crystals may form cubes or rectangular boxes, such as halite shown at right) orhexagonal crystal system (where the crystals may form hexagons, such as ordinary water ice).

SOURCE: WIKIPEDIA