Crystal is any solid material in which the component atoms are arranged in a definite pattern and whose surface regularity reflects its
internal symmetry.
A crystal structure
is formed only when the group of atoms is arranged identically at the lattice
point. The group of atoms or molecules is called a basis. Lattice point is
actually an imaginary concept. Periodic arrangement of infinite point on a
plane without atoms is called lattice. When we place identical groups of atoms into lattice then it is called basis.
In other way we can say that-
basis + lattice = crystal.
Line joining any
two points is a translation in lattice. Two non-collinear translation leads to
plane lattice and three non-coplanar translation leads to a space lattice.
If a liquid is
allowed to cool slowly, the molecules will arrange themselves in an orderly manner and this will finally result in a crystalline solid.