Classification of Solids

Solids are broadly classified into two types

(A) Crystalline solids   (B) Amorphous solids.

  • A crystalline solid is a substance whose constituent particles possess regular orderly arrangement e.g. Sodium chloride, sucrose, etc.
  • An amorphous solid is a substance whose constituent particles do not possess a regular orderly arrangement e.g. glass, plastics, rubber, proteins etc.
  • Though amorphous solids do not possess long range regularity, in some cases they may possess small regions of orderly arrangement. These crystalline parts of an otherwise amorphous solid are known as crystallites.
  • An amorphous solid does not possess a sharp melting point. It undergoes liquefication over a broad range of temperature. However crystalline solids have sharp melting points
  • Amorphous substances are also, sometimes, referred to as super cooled liquids because they posses disorderly arrangement like liquids. In fact many amorphous solids such as glass are capable flowing. Careful examination of the window panes of very old houses reveals that the panes are thicker at the bottom than at the top because the glass has flown under constant influence of gravity. 

Distinction between crystalline and amorphous solids

S.No.      Crystalline solids   Amorphous solids
1.The arrangement of constituent particles is regular so they possess definite and regular geometry1. The arrangement of constituent particles is irregular. Thus they do not have any definite geometry. 
2.They have sharp melting pointsThey do not have sharp melting points
3.There is regularity in the external form when crystals are formedThere is no regularity in the external form when amorphous solids are formed
4.Crystalline solids are regarded as true solidsAmorphous solids are regarded as super cooled liquids or pseudo solids
5.Crystalline solids give a regular cut when cut with a sharp – edged knifeAmorphous solids give irregular cut.
6.Crystalline solids are anisotropic. This implies that physical properties such as refractive index, conductivity, thermal expansion etc are different in different directions. This is due to orderly arrangement of particlesAmorphous solids are isotropic in nature. This implies that various physical properties are same in all the directions. This is because of random arrangement of particles.

Classification of crystalline Solids on the Basis of Forces of attraction

Crystal ClassificationUnit ParticlesBinding ForcesPropertiesExamples
AtomicAtomsLondon dispersion forcesSoft, very low melting, poor thermal and electrical conductorsNoble gases
MolecularPolar 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 conductorsDry ice (solid, methane
IonicPositive and negative ionsIonic bondsHard and brittle, high melting points, high heats of fusion, poor thermal and electrical conductorsNaCl, ZnS
CovalentAtoms that are connected in covalent bond networkCovalent bondsVery hard, very high melting points, poor thermal and electrical conductorsDiamond, quartz, silicon
MetallicCations in electron cloudMetallic bondsSoft to very hard, low to very high melting points, excellent thermal and electrical conductors, malleable and ductileAll metallic elements, for example, Cu, Fe, Zn