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 geometry	1. The arrangement of constituent particles is irregular. Thus they do not have any definite geometry.
2.	They have sharp melting points	They do not have sharp melting points
3.	There is regularity in the external form when crystals are formed	There is no regularity in the external form when amorphous solids are formed
4.	Crystalline solids are regarded as true solids	Amorphous solids are regarded as super cooled liquids or pseudo solids
5.	Crystalline solids give a regular cut when cut with a sharp – edged knife	Amorphous 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 particles	Amorphous 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 Classification	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