Matter exists in three different states like solid, liquid and gas. These three states of matter differ from one another because of the difference in the attractive forces and separating forces.

Solids: Solids are the substances which have definite shapes, definite volumes and show non-compressibility, high rigidity, large mechanical strength etc.

A solid may be crystalline or amor­phous.

Crystalline Solids have the following properties:

(a) Definite shapes or geometries

(b) Long range order

(c) Sharp melting points

Different types of Crystal geometries and shapes. A solid is defined as that form of matter which possesses rigidity and hence possesses a definite shape and a definite volume. Unlike gases and liquids in which the molecules are free to move about and hence constitute fluid state, in a solids the constituent particles are not free to move but oscillate about their fixed positions.

Crystal. It is a piece of solid substance with a regular geometrical shape caused by a regular arrangement of atoms, ions or molecules making up the solid.

Crystalline. The solid which has regular arrangement of atoms, ions or molecules is called crystalline solid.

Crystallography. The study of geometry and symmetry of crystal is called crystallography.

Lattice. A well-ordered and regular arrangement of atoms, molecules or ions in the three dimensional space is called crystal lattice. If the particles are replaced by points, the crystal structure would be an array of points which is called lattice.

These points are called lattice points, and the lattice is called space lattice. There are five types of two dimensional lattices. They are hexagonal, square, rectangular, rhombic and parallelogram lattices.