Ionic Solids

In ionic solids, the constituent particles are ions of opposite charges. Each ion is surrounded by a definite number of ions of opposite charge. The number of ions that surround a particular ion of opposite charge its called co – ordination number of the ion. For example, in sodium chloride crystal each sodium ion is surrounded by six chloride ions. Hence coordination number of  is 6. At the same time each chloride ion is surrounded by six  ions. Therefore the co – ordination number of ion is also 6. However, in calcium fluoride crystal each  ion is surrounded by eight fluoride  ions and each  ion is surrounded by four  ions. Thus, in crystal co – ordination numbers of  and  ions are respectively 4 and 8. The interparticle forces in ionic solids are ionic bonds operating between the ions of opposite charges some examples of ionic solids are : sodium chloride (NaCl) ; ceasium chloride (CsCl), zinc sulphide (ZnS), calcium fluoride, etc.

Characteristics of Ionic Solids

Some common characteristics of ionic solids are as follows:

They are hard, brittle and have low volatility.

They have high melting points.

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

They are generally soluble in polar solvents like water.

Covalent Solids

In these types of solids the constituent particles are atoms of same or different elements connected to each other by covalent bond network. For example, in diamond only carbon atoms constitute the covalent network while carborundum covalent bond network is constituted by silicon and carbon atoms. Obviously, the interparticle forces operating in these solids are covalent bonds. These solids are also called network solids because the covalent bonds extend in three dimensions forming a giant interlocking structure. Some examples of covalent solids are :

Diamond, silicon carbide, aluminium nitrite etc.

Characteristics of Covalent Solids

Some common characteristics of covalent solids are :

• They are very hard. Diamond is the hardest naturally occurring substance.

• They have very high melting points.

• They are poor conductors of heat and electricity.

• They have high heats of fusion.

Metallic Solids

In these type of solids, the constituent particles are metal atoms. The interparticle forces in these solids are metallic bonds. In the metallic crystals the metal atoms occupy the fixed positions but their valence electrons are mobile. The close packed assembly of metal kernels (part of metal atom without valence electrons) remain immersed in the sea of mobile valence electrons. The attractive force between the kernels and mobile valence electrons is termed as metallic bond.

Characteristics of Metallic Solids

The common characteristics of metallic solids are as follows:

• They generally range from soft to very hard.

• They are malleable and ductile.

• They are good conductors of heat and electricity.

• They possess bright lustre.

• They have high melting and boiling points.

• They have moderate heats of fusion.

• The summary of classification of solids on the basis of interparticle forces is given in

Classification of Solids on the Basis of Binding Forces

CrystalClassification	Unit Particles	Binding Forces	Properties	Examples
Atomic	Atoms	London dispersion forces	Soft, very low melting, poor thermal and electrical conductors	Noble gases
Molecular	Polar or  non – polar molecules	Vander Waal’s forces (London dispersion, dipole – dipole forces hydrogen bonds)	Fairly soft, low to moderately high melting points, poor thermal and electrical conductors	Dry ice (solid, methane
Ionic	Positive and negative ions	Ionic bonds	Hard and brittle, high melting points, high heats of fusion, poor thermal and electrical conductors	NaCl, ZnS
Covalent	Atoms that are connected in covalent bond network	Covalent bonds	Very hard, very high melting points, poor thermal and electrical conductors	Diamond, quartz, silicon
Metallic	Cations in electron cloud	Metallic bonds	Soft to very hard, low to very high melting points, excellent thermal and electrical conductors, malleable and ductile	All metallic elements, for example, Cu, Fe, Zn