Solid State

Solids are characterized by their high density and low compressibility as compared to those of the gases. The properties of solids indicate that the molecule (or ions) in them are relatively close together.

Solids can be broadly classified into two categories - crystalline and amorphous solids.

Crystalline Solids: The characteristics of a crystal are its sharp melting point, its flat faces and sharp edges. These properties are due to a high degree of internal order which extends throughout the crystal (a definite pattern constantly repeating in space). This is know as long-range order.

Amorphous Solids: Amorphous solids do not have the long-range order but have a short-range order. This characteristics may not be found around a similar atom placed at a distance from the other atom. Examples of amorphous solids are glass, used silica, rubber and polymers.

Amorphous solids do not have the characteristics as possessed by crystalline solids. In many ways, they are more closely related to liquids than to the crystalline solids and are, therefore, regarded as supercooled liquids with high viscosity.

Classification of Crystals

Molecular Crystals (or van der Waals Crystals): Molecular crystals are those in which the crystalline state is composed of an aggregate of discrete molecules held together by van der Waals forces. Because of these weaker forces, molecular crystals are soft and posses comparatively low melting points. Examples are CO2, CCl4, Ar and most of the organic compounds.

Ionic Crystals: Ionic crystals involve electrostatic forces amongst their structural units. Because of stronger forces, ionic crystals are strong and likely to be brittle. The melting points are high, which decease with increasing size of the ions. In ionic crystals, some of the atoms may be held together by covalent bonds to form ions having definite positions and orientations in crystal lattice.

Covalent Crystals: Covalent crystals involve forces of chemical nature (covalent bonds) extended in three dimensions. These forces are strong, and consequently the crystals are strong and hard, with high melting points. Examples are diamond, silicon

Metallic crystals: Electrons are held loosely in these type of crystals. They are god conductors or electricity. Metallic crystals are strong and can be bent.

Hydrogen bonded crystals: Molecules having H atom bonded to more electronegative atoms like F, O, N exhibit strong hydrogen bonding. Crystals of H2O, HF, NH3 and amino acids are some examples of hydrogen bonded crystals.

Bravais Lattices

Bravais showed that there are 14 different possible kinds of three dimensional lattices. The geometric shape of the crystal lattice must be same as that of solid crystal itself. For example, if unit lattice is having cubical structure the crystal is also a cube.

These 14 Bravais lattices are grouped into seven crystal system based on unit cell symmetry.

  1. Cubic
  2. Tetragonal
  3. Rhombic
  4. Hexagonal
  5. Trigonal
  6. Monoclinic
  7. Triclinic