Friday, August 5, 2011

Classification of Crystals

The crystals are classified into four types:

(I) atomic crystals

(Ii) ionic crystals

(Iii) covalent crystals

(Iv) molecular crystals

This classification is based on the nature of forces and bonds between atoms, ions or molecules constituting the crystal.

(I) atomic crystals

Examples of these crystals are metals, so they are also called metal crystals. They are made of atoms packed and held together by metallic bonds. In metals, the valence electrons move freely in the crystal lattice and said that from an electron gas, where the positive ions are immersed. These crystals are characterized by: (a) Lustre (b) the high melting point (c) the electrical and thermal conductivity (d) ductility (drawing paper) (e) ductility (from to son).

(Ii) ionic crystals

These crystals consist of positively and negatively charged ions, held together by electrostatic forces of attraction. Individual ions

has no identity. They are characterized by: (a) The high melting point (b) of the conduction of electricity in the molten state and in solution. (C), accuracy and hardness (d) crystal growth without restriction until it is broken or has stopped growing.

(Iii) covalent crystals

These crystals are composed of atoms linked by covalent bonds. Covalent bonds are very strong. They need large amounts of energy to break them. Examples of covalent crystals are: graphite, diamond, silica and carborundum. In diamond, each carbon atom is connected to four other carbon atoms at an angle of 109 degrees, called the tetrahedral angle. Because of the close packing of atoms and the large number of covalent bonds, the diamond is very hard and has a high melting point.

In graphite, each carbon atom is connected to three carbon atoms at an angle of 120 degrees, forming layers of hexagons. Areheld adjacent layers together by weak Vander Waals forces. Due to the space between layers of flexible graphite layers slide easily over each other to

Graphite is a greasy texture. Graphite conducts electricity by air layers, because each carbon is one of the free electron. Covalent crystals are characterized by:

(A), high melting point (b) LD (c) the high refractive index

(Iv) in molecular crystals

These crystals are composed of molecules, such as ice, I <sub>2,</sub> CO <sub>2</sub> molecules are held together by forces following:

Hydrogen bonding (a) (which is a force of electrostatic attraction between a partially positive hydrogen atom of a molecule and an electronegative atom other molecules).

(B) A weak Vander Waal forces (due to the attraction between the atomic nuclei and electrons from one molecule to other molecules). Such

crystals are characterized by:

(A) The low melting point (b) no - the conduction of heat and electricity.


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