Classification of crystalline solids 👩‍🔬

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 Crystalline solids are classified into four categories: ionic solids, covalent network solids, molecular solids and metallic solids.

1. Ionic crystals:

                                                    Ionic crystals have the following characteristics:

  1. The constituent particles of ionic crystals are charged ions. The cations and anions may differ in size.
  2. Each ion of a given sign of charge bonded to ion of opposite charge around it by columb force. In other words, the particles of ionic crystals are held by electrostatic force of attraction between oppositely charged ions.
  3. Ionic crystals are hard and brittle. They have high melting  points. 
  4. These are nonconductors of eletricity in solid state. However, they are good condyctors when melted or dissolved in water.
For example:NaCl, K2SO4, CaF2, KCl are ionic crystals. 



2.Covalent network  crystals:                         Characteristics of covalent network crystals are as follows :

  1. The constituent particles in covalent network solids are atoms.
  2. The atoms in this crystals are linked by a continuous system of covalent bonds. The result is a rigid three dimensional network that forms a giant molecule. The entire crystal is a single molecule. 
  3. As a result of rigid and strong bonded structure, covalent network crystals are very hard. In fact they are the hardest and most incompressible of all the materials. These crystals have high melting and boiling points. 
  4. The electrons are localised in covalent bond and hence are not mobile. As a result, covalent solids are poor conductors of heat and electricity.
For example: duamond, quartz(SiO2), boron nitride, carborandum are covalent network solids.


3. Molecular solids:
                    Substances such as Cl2, CH4, H2, CO2, O2 on solidification give molecular crystals. Crystalline organic compounds are also molecular solids.
  1. The constituent particles of molecular solids are molecules of the same substance. 
  2. The bond within the molecules are covalent. The molecules are held together by various intermolecular forces of attraction.                                 💎Weak dipole-dipole interaction in polar molecules such as solid HCl, H2O, SO2, which posses permanent dipole moment.                                       💎Very weak dispersion or Londen forces in nonpolar molecules such as solid CH4, H2.                                          💎Intermolecular hydrogen bonding in solids such as H2O, NH3, HF and so forth. 
  3. Because of weak intermolecular attraction forces, molecular solids are usually soft substances with low melting points.
Thes solids are poor electrical conductors and are good insulators.





4. Metallic crystals:
                                  These are crystalline solids formed by atoms of of the same mettallic elements, held together by a metallic bond.
Metallic crystals have following properties:
  1. Metals are malleable, that is, they can be hammered into thin sheets.
  2. Metals are ductile, that is they can be drawn into wires.
  3. Metals have good electrical and thermal conductivity.
Examples: metals such as Na, K, Ca, Li, Fe, Co, etc.


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