The vast majority of solids is in the crystalline state. Crystals called solids which have a periodic arrangement of proper particles (atoms, molecules or ions) that is characterized by long-range order (a three-dimensional periodic structure throughout the volume of the solid body). The ordered structure of the substance gives a lower internal energy. The crystalline state of matter is thermodynamically more stable than amorphous, so the material in this condition is very common in nature.
However, there are solids which an amorphous state is thermodynamically more favorable than the crystal (plastic, organic glass, etc.). Sophisticated specific form of macromolecules such substances does not energetically favorable periodically proper packaging structure. The geometric shape of the crystals variuos and depending on the nature and particle size, crystal form, form of chemical bonds between them and other factors. The external form of the crystal is the result of its internal structure. Specific location of the particles in space that makes the structure and properties of the crystal,called crystal& grid. Crystal characterized by energy, crystal& grid constant, and the coordination number. Crystal grid energy, referred to as the energy required to break one mol crystal particles and its removal generators beyond their interaction. The value energy crystal grid is a measure the strength of crystal than it is higher, the stronger the crystal.Crystal grid constant& it is the distance between the centers of particles in the crystal lattice in the direction of the characteristic axes. Coordination number(cn) is called the number of particles is directly adjacent to the given particle in a crystal. Cn can range from 2 to 12, depending on the structure of the outer energy levels of the atoms forming the crystal. The larger the coordination number, the tighter packing of the crystal. Some substances similar chemical nature, e.g., Ca2SiO4 and Mg2SiO4 same form crystalline structures. Such substances are called isomorphic. Crystal lattice isomorphic substances differ unusual mobility and interchangeability of the individual components, it is possible if the sizes of the particles forming the lattice do not differ by more than 15%.
Many of the simple substances or the compounds can exist in two or more crystal structures. This phenomenon is called polymorphism (example - allotropes of carbon: diamond, graphite, carbyne, fullerenes). Manifestation polymorphism observed in the case where one and the same atom has a different degree of excitation of valence electron orbitals and, consequently, the degree of hybridization.
& Depending on the nature of particles in the crystal lattice, and the nature of communication between all the crystals divided into molecular, atomic, covalent, ionic, and metal . Furthermore, there are mixed crystals chemical properties.For more information read about crystals and its structure.