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Atomic imperfections / point defect When deviations exist from the regular arrangement around an atom or a group of atom in a crystalline substance the defects are called point defects. Types of point defects (A) Stoichiometric defects If imperfection in the crystal are such that the ratio between the cations and the ratio between the cations and anions remains the same as represented by the molecular formula, the defects are called Stoichiometric defects (for CSIR-NET Coaching in Chemistry , JAM Coaching in Chemistry These are 2 types 1. Schottky defects In an ionic crystal of A + B - types equal number of cations and anions are missing from their lattice site so that the electrical neutrality is maintained it is called schottky defects. The schottky defects containing one pair of holes due to missing of one cation and one anion is shown in Fig   Compound exhibiting schottky defects have High coordination number Small difference in size of cation

Solids can be classified on the basis of nature of order in the arrangement of constituent particles (atoms, ions or molecules) into two types as  1. Crystalline solids 2. Amorphous solids 1 . Crystalline solids (Topic for CSIR-NET Coaching in Chemistry) The substance whose constituent particles (atoms, ions or molecules) are arranged in a defined geometric pattern in three dimensional space are called crystalline solids. X-ray diffraction studies of these crystalline solids reveal that in these solids, the constituents (atoms, ions or molecules) are arranged in a definite orderly arrangement. The regular arrangement of the constituents extends throughout the three dimensional network of crystals. In other words, the crystalline substance are said to have Long range order. This means that in crystalline solids, there is a regular pattern or arrangement of particles which repeats itself periodically aver the entire crystal. In fact a crystalline solid consists of a large