Crystalline solids can be classified into different categories depending upon the type of constituent particles and the nature of attractive forces operating between them.
Various categories of crystalline solids are:
1. Molecular solids
2. Ionic solids
3. Covalent solids
4. Metallic solids.
1. Molecular Solids
In these solids, the constituent particles which pack up together are molecules of the substance. These molecules may be non-polar (dipole moment = 0) such as CO2, I2 CC14, etc.
or they may be polar (dipole moment> 0) like H2O, HCl, etc.
(i) In case of non-polar molecules, the attractive forces operating between the molecules are van der Waal forces (also called dispersion forces). The examples of such solids are: dry ice (solid CO2), iodine, etc.
(ii) In case of polar molecules, the attractive forces operating between the molecules in solid state are dipole dipole forces. The examples of such solids are: solid SO2, solid HCI. However, in some solids with polar molecules, the interparticle forces are hydrogen bonds. The examples of such solids are iceH2O); solid hydrogen fluoride (HF); solid ammonia NH3), etc.
Characteristics of Molecular Solids
Some of the general characteristics of molecular solids are:
(i) They are generally soft.
(ii) Their melting paints are low to moderately high. The melting points of solids with non-polar molecules are relatively low whereas solids with polar molecules have moderately high melting points.
(iii) They have low enthalpies of vaporization and hence are volatile.
(iv) They are generally bad conductors of heat and electricity.
(v) They have generally low density.
2. 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 is called co-ordination number of the ion. For example, in sodium chloride crystal each sodium ion (Na+) is surrounded by six chloride ions. Hence, co-ordination number of Na+ is 6. At the same time each chloride ion is surrounded by six Na+ ions. Therefore, the co-ordination number of Cl- ion is also 6. However, in calcium fluoride crystal each Ca2+ ion is surrounded by eight fluoride (F -) ions and each F – ion is surrounded by four Ca2+ ions. Thus, in CaF2 crystal, co-ordination numbers of Ca2+ and F- ions are respectively 8 and 4. 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 (CaF2), etc.
Characteristics of Ionic Solids
Some common characteristics of ionic solids are as follows:
(i) They are hard and brittle.
(ii) They have high enthalpies of fusion and vaporisation. Therefore, they possess high melting points and low volatility.
(iii) They are poor conductors of electricity in solid state, however, they become good conductors of electricity in molten state or in dissolved state.
(iv) They are generally soluble in polar solvents like water.
3. Covalent or Network Solids
In this type of solids the constituent particles are atoms of same or different elements connected to each other by covalent bond network. For example,. in cliamond only carbon atoms constitute the covalent network while in 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, silica. etc.
Characteristics of Covalent Solids
Some common characteristics of covalent solids are:
(i) They are very hard and brittle. Diamond is the hardest substance that occurs in nature.
(ii) They have very high melting points.
(iii) They are poor conductors of heat and electricity.
(iv) They have high enthalpies of fusion.
It may be mentioned here that graphite is also a network solid but it shows exceptional properties due to its typical structure. Grarhite is a soft solid and is used as lubricant. It is also good conductor of electricity and is used for making electrodes.
4. Metallic Solids
In this 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:
(i) They generally range from soft to very hard.
(ii) They are malleable and ductile.
(iii) They are good conductors of heat and electricity.
(iv) They possess bright lustre.
(v) They have high melting and boiling points.
(vi) They have moderate enthalpies of fusion.
The summary of classification of solids on the basis of interparticle forces is given in Table 14.1.
Table 14.1. Classification of Solids on the Basis of Binding Forces
Polar or non –polar molecules
Positive and negative
Atoms that are
connected in covalent
Cations in electron
van der Waals’
dispersion, dipole dipole
Fairly soft, low to moderately high melting points, poor thermal and electrical conductors
Hard and brittle, high melting points, high enthalpies of fusion,
poor thermal and electrical conductors
Very hard, very high melting points,
poor thermal and electrical conductors
Soft to very hard, low to very high melting points, example thermal and electrical conductors, malleable
Dry ice (solid CO2), methane ( CH4) argon , chloroform ( CHCl3), iodine (I2)
NaCl, ZnS, MgO, CaF2, KNO3.
Diamond, quartz, silica,
All metallic elements, for example, Cu, Fe, Zn, Ag, Au.