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  Types of Crystals

Types of Crystals:

Crystals are categorized into four types depending upon the nature of the units that occupy the lattice points.

1. Molecular Crystals
2. Covalent Crystals
3. Metallic Crystals
4. Ionic Crystals

1) Molecular Crystals:

The lattice points in molecular crystals comprise of molecules that do not carry any charge. The forces binding the molecules altogether are of two kinds:

(i) Dipole-dipole interaction and
(ii) Vanderwaal’s forces.

Dipole-dipole forces take place in solids that consists of polar molecules example, ice. The Vanderwaal’s forces are more common and take place in all kinds of molecular solids.

2) Covalent Crystals:

The lattice in covalent crystals comprises of atoms connected together by a continuous system of covalent bonds. Diamond is a good illustration for this type.

3) Metallic Crystals:

Metallic crystal comprises of an assemblage of positive ions immersed in a sea of mobile electrons. Therefore, each electron belongs to a number of positive ions and each positive ion belongs to a number of electrons. The force which binds a metal ion to a number of electrons in its sphere of influence is termed as metallic bond. This force of attraction is strong and is therefore responsible for a compact solid structure of metals.

4) Ionic Crystals:

In ionic crystals, the units taking up lattice points are positive and negative ions. Each ion of a specified sign is held by coulombic forces of attraction to all ions of opposite sign. The forces are much strong. The ionic crystals have the characteristics shown below:

a. The heats of vaporization of ionic crystals are very high.
b. The vapor pressure of ionic crystals at normal temperature is very low.
c. The melting and boiling points of ionic crystals are too high.
d. Ionic crystals are brittle and hard.
e. Ionic crystals are insulators in solid state.
f. Ionic crystals are soluble in water and as well in other polar solvents.
g. Ionic solids are good conductors whenever dissolved in water.

 

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