Classification of Solid State

Amorphous and Crystalline Solids

On the basis of the nature of order present in the arrangement of their constituent particles, Solids can be classified as:

· Amorphous Solid

· Crystalline Solids

Crystalline Solids

A crystalline solid usually consists of a large number of small crystals, each of them having a definite characteristic geometrical shape. The arrangement of constituent particles (atoms, molecules or ions) in a crystal is ordered and repetitive in three dimensions.

Crystal has a long range order which means that there is a regular pattern of arrangement of particles which repeats itself periodically over the entire crystal. Sodium chloride and quartz are typical examples of crystalline solids.

Amorphous Solid

Glass, rubber and many plastics do not form crystals when their liquids solidify on cooling. These are called amorphous solids. The term amorphous comes from the Greek word amorphos, meaning no form. The arrangement of constituent particles (atoms, molecules or ions) in such a solid has only short range order.

Two dimensional structure of (a) quartz and (b) quartz glass

Distinction between Crystalline and Amorphous Solids

Classification of Crystalline Solid

We will classify crystalline solids on the basis of nature of intermolecular forces or bonds that hold the constituent particles together. These are — (i) Van der waals forces; (ii) Ionic bonds; (iii) Covalent bonds; and (iv) Metallic bonds. On this basis, crystalline solids are classified into four categories viz.,

· Molecular solids

· Ionic solids

· Metallic solids

· Covalent solids

Molecular Solids

Molecules are the constituent particles of molecular solids. These are further sub divided into the following categories:

(i) Non polar Molecular Solids:

Ø They comprise of either atoms, for example, argon and helium or the molecules formed by non polar covalent bonds.

Ø Example: H₂, Cl₂ and I₂.

Ø In these solids, the atoms or molecules are held by weak dispersion forces or London forces.

Ø These solids are soft and non-conductors of electricity.

Ø They have low melting points and are usually in liquid or gaseous state at room temperature and pressure.

(ii) Polar Molecular Solids:

Ø The molecules of substances like HCl, SO₂, etc., are formed by polar covalent bonds.

Ø The molecules in such solids are held together by relatively stronger dipole-dipole interactions.

Ø These solids are soft and non-conductors of electricity.

Ø Their melting points are higher than those of non polar molecular solids yet most of these are gases or liquids under room temperature and pressure.

Ø Example: Solid SO₂ and solid NH₃.

(iii) Hydrogen Bonded Molecular Solids:

Ø The molecules of such solids contain polar covalent bonds between H and F, O or N atoms.

Ø Strong hydrogen bonding binds molecules of such solids like H₂O (ice).

Ø They are non-conductors of electricity.

Ø Generally they are volatile liquids or soft solids under room temperature and pressure.

Ionic Solids

Ions are the constituent particles of ionic solids. Such solids are formed by the three dimensional arrangements of cations and anions bound by strong coulombic (electrostatic) forces.

Characteristics of Ionic Solids

Ø They are hard, brittle and have low volatility.

Ø They have high melting points.

Ø They are poor conductors of electricity in solid state, however they become good conductors of electricity in molten state or in dissolved state.

Ø They are generally soluble in polar solvents like water.

Metallic Solids

                  Metallic solids are solids composed of metal atoms that are held together by metallic bonds. These bonds are like huge molecular orbitals that span across the whole solid. This means the electrons in metallic solids are delocalized. 

Characteristics of Metallic Solids

Ø They generally range from soft to very hard.

Ø They are malleable and ductile.

Ø They are good conductors of heat and electricity.

Ø They possess bright lustre.

Ø They have high melting and boiling points.

Ø They have moderate heats of fusion.

Covalent or Network Solids

It is a chemical compound (or element) in which the atoms are bonded by covalent bonds in a continuous network extending throughout the material. In a network solid there are no individual molecules, and the entire crystal or amorphous solid may be considered a macromolecule.

Characteristics of Covalent Solids

Ø They are very hard.

Ø Diamond and silicon carbide are typical examples of such solids.

Network structure of diamond

Ø Although Graphite also belongs to this class of crystals, but it is soft and is a conductor of electricity. Its exceptional properties are due to its typical structure.

Structure of graphite

Ø They have very high melting points.

Ø They are poor conductors of heat and electricity.

Ø They have high heats of fusion.

Different properties of the four types of solids