The crystal structure is formed by associating every lattice point with an assembly of atoms or molecules or ions, which are identical in composition, arrangement and orientation, is called as the basis. The atomic arrangement in a crystal is called crystal structure. If the basis is substituted for the lattice points, then the resulting structure is called crystal structure as shown in Fig. 2.2. Thus lattice + basis = crystal structure. The basis shown in Fig. 2.2 contains three different atoms. In copper and sodium crystals the basis is single atoms; in NaCl, the basis is diatomic and in CaF2 the basis is triatomic.
Figure 2.2 Two-dimensional crystal structure