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 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|