Explain how a crystalline substance is different from amorphous or else.How to classify substances as a thumb rule into the two categories.
Any solid which has both translational and rotational symmetry is classified as a crystalline material. If these two conditions are not satisfied, then the solid is either quasi-crystalline or amorphous.
First, the terms can be understood as the following.
Translational symmetry can be described as the repetition of a structural unit (or an atom) over some characteristic distance. For instance, in a simple cubic material, along different crystallographic directions, the characteristic distance would be multiples of the lattice parameter. If, along the direction, an atom is seen at steps of 'a' then along an atom would be repeated for every root_3 X a.
Rotational symmetry is defined a unit is repeated on angular rotation of the lattice. The simplest example is that of the hexagonal crystal having rotational symmetry for every 60 degrees when its rotated along the c-axis.
Amorphous Materials, on the other hand, have no such notion of long range symmetry or order. They do possess short range order to an extent but even these short range ordered units are not repeatable over the entire structure
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