Engineering Chemistry S Datta

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Solid State Chemistry
Solids are mainly characterised by their definite shapes and considerable mechanical strength
and rigidity. The rigidity arises due to the absence of the translatory movement of the structural
units (like atoms, ions etc.) of the solid. These units remain fixed to a mean position about
which they may undergo vibration. The forces of attraction between these units are large
enough.
Solids are generally classified under two heads, namely crystalline and amorphous.
In a crystalline solid, the constituent atoms are arranged in a definite pattern constantly
repeated and in consequence giving rise to a definite geometrical configuration of crystalline
solids. The crystals often have a plane fracture and they also have sharp melting points. On
the other hand the amorphous solids have no definite geometrical shape. The amorphous solids
melt slowly over a temperature range.
CRYSTAL
The term crystal originates from the Greek word ‘‘Krystallos’’ that means clear ice, and
was first applied to transparent quartz stone with a wrong assumption that these stones were
formed from water by extreme cold. In the 17th century the name crystal was extended to
other solids possessing manifestations of solids that bore the relation to the original rock crystals
of being bounded by many flat, shiny faces, arranged symmetrically.
A crystal grows from melt or a solution or from gaseous state by the deposition of atoms
or ions onto its surfaces by which crystals are bounded. The surfaces are referred to as faces
and an edge is formed where two surfaces intersect. The angle between the normals to the two
intersecting faces is called the interfacial angle.
In a crystal the atoms or ions are arranged like soldiers on a parade ground in well
defined columns and rows and this regularity of atomic arrangements in a crystal is the basic
feature of crystals. There is a basic unit in any real crystal and when the basic unit is
systematically repeated, reproduces the whole structure. Thus the first notable feature of the
regularity of the crystal structure is the periodicity of their patterns and a crystal is therefore
a periodic array of atoms and molecules in space.
The various modes of arrangement of atoms of molecules in space to satisfy the need of
periodicity are governed by some specific rules and principles which lead to the development
of the subject of crystallography.
In a more specific way a crystal may be defined as a solid substance having a
definite geometrical shape with flat faces and sharp edges.
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