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Crystalline Solids: Metallic and Covalent Solids

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Crystalline Solids: Metallic and Covalent Solids - Lesson Summary

Crystalline solids are broadly classified into molecular solids, ionic solids, metallic solids, and covalent or network solids.

Metallic solids: 
In metallic solids the constituent particles in a metallic solid are positive ions in a sea of delocalised electrons.
 
The electrostatic forces of attraction between the positive ions and the electron cloud constitute the metallic bonding, which holds the positive ions together. These electrons are not localised on any atom. These free and mobile electrons make metallic solids good conductors of electricity and heat. Metals are highly malleable and ductile. Metallic solids have fairly high melting and boiling points.
 
Covalent or network solids: 
The constituent particles in these solids are atoms. These atoms are held together in large networks by covalent bonds. Covalent solids are also called network solids. These are also termed as giant molecules.
EX: Diamond, graphite and quartz are some network solids.

These solids are very hard, and possess very high melting points. These solids are non-conductors of electricity, except for graphite.
 
Diamond: 
In diamond, each carbon atom is bonded tetrahedrally to 4 other carbon atoms. a regular tetrahedral arrangement of carbon atoms bonded to one another in a three-dimensional manner forms a network, resulting in a rigid giant molecule. Diamond is the hardest substance known, and has a melting point of 3550 0c.
 

 
Graphite: 
In graphite, each carbon atom is bonded to three other carbon atoms, resulting in hexagonal rings, which, when repeated throughout a plane, form sheets or layers.
 

 
As each carbon atom in graphite is bonded to only 3 other neighbouring carbon atoms, the 4 th valence electron is free to move about in different layers. The free, mobile electrons account for the exceptional electrical conductivity in graphite.

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