Intermolecular forces and thermal energy are the two factors on which physical states of matter depend. While the intermolecular forces of attraction tend to keep the particles closer; the thermal energy tends to keep the particles apart from each other by making them move faster.
When the net resultant of these two opposing forces, i.e. intermolecular forces and thermal energy, makes the particles cling together and forces them to occupy fixed positions, matters exist in solid state.
Solids can be classified into two types on the basis of the arrangements of their constituent particles (atoms, molecules or ions). These two types are Crystalline Solid and Amorphous Solid.
Solids having large number of crystals; each with definite characteristic geometrical shape; are called crystalline solids.
The constituent particles of crystalline solid are arranged in regular pattern which is repeated periodically over the entire crystal. Such type of arrangement is called long range order. Crystalline solids are anisotropic in nature, i.e. many physical properties, such as electrical resistance, refractive index, etc. are different along different axes. Crystal of NaCl, Quartz, Ice, HCl, Iron, etc. are some examples of crystalline solid.
Solids having irregular shapes of particles are known as Amorphous Solids. The word ‘Amorphous’ came from Greek ‘Amorphos’ which means no shape.
The constituent particles of amorphous solids have only short range order of arrangement, i.e. regular and periodical arrangement of particles is seen to a short distance only. The structures of amorphous solids are similar to that of liquids. Glass, rubber, plastics, etc. are some of the examples of amorphous solids. Amorphous solids are isotropic in nature, i.e. physical properties of amorphous solids are same in all directions.
In old buildings, it is often seen that glasses of windows get slightly thickened at bottom, this happens because glass which is an amorphous solid; flows down very slowly. Some very old glasses get milky appearance because of some crystallization. This happens because on heating, glasses get crystallized at some temperature. This is the cause; amorphous solids are also known as Pseudo Solids or Super Cooled Liquids.
Amorphous solids are very useful but most of the solids are crystalline in nature. Crystalline solids are classified into four types; based on the intermolecular forces operating in them.
Molecular Solids Solids having molecules as their constituent particles are called Molecular solids. For, example, Hydrogen, Chlorine, Water, HCl, solid carbon dioxide, sucrose, etc.
Molecular solids are classified into three types on the basis of their bond:
(a) Non Polar Molecular Solids Solids which are comprised of atoms only, such as helium and argon or molecules; formed because of the non polar covalent bonds are known as Non-Polar Molecular Solids. For example – H2, Cl2, I2, etc.
(b) Polar Molecular Solids The solids which are formed by polar covalent bonds are known as Polar Molecular solids. For example – HCl, SO2, NH3, etc
(c) Hydrogen bonded Molecular Solids The molecules of hydrogen bonded molecular solids contain polar covalent bond between H and O, F or N. In solids such as H2O (ice) molecules are bound together strongly with hydrogen bond. HF, H2O (ice), etc are the examples of hydrogen bound molecular solids.
Ionic Solids Solids, in which ions are the constituent particles, are called ionic solids. These solids are formed because of three dimensional arrangements of cations and anions bound together with strong electrostatic forces (coulombic forces). For example NaCl.
Metallic Solids All metals are referred as Metallic solids. Their constituent particles are positive ions. These positive ions are surrounded by free moving electrons. For example – iron, aluminium, etc.
Covalent Solids Crystalline solids are formed by non metals because of formation of covalent bonds between the adjacent molecules throughout the crystal. These are also known as Network Solids. These are also called giant molecules. For example – diamond, graphite, silicon carbide, etc.
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