# Crystalline state

Crystalline state

“A crystal is a solid composed of atoms (ions or molecules) arranged in an orderly repetitive array”.

Most of the naturally occurring solids are found to have definite crystalline shapes which can be recognized easily. These are in large size because these are formed very slowly, thus particles get sufficient time to get proper position in the crystal structure. Some crystalline solids are so small that appear to be amorphous. But on examination under a powerful microscope they also seen to have a definite crystalline shape. Such solids are known as micro crystalline solids.

Thus the crystallinity of a crystal may be defined as “A condition of matter resulting from an orderly, cohesive, three dimensional arrangement of its component particles (atoms, ions or molecules) in space”. This three dimensional arrangement is called crystal lattice or space lattice. The positions occupied by the particles in the crystal lattice are called lattice sites or lattice points. The lattices are bound by surfaces that are usually planar and are known as faces of the crystal.

“The smallest geometrical position of the crystal which can be used as repetitive unit to build up the whole crystal is called a unit cell”.

The angle between two perpendiculars to the two intersecting faces is termed as the interfacial angle which may be same as the angle between the unit cell edges.

Goniometer is used to measure the interfacial angle. It is important to note that interfacial angle of a substance remains the same although its shape may be different due to conditions of formation. This is known as law of constancy of interfacial angle or law of crystallography.

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