Crystal Defects and Non-Stoichiometry

September 13, 2026

| Chemical Science Exams, Inorganic Chemistry

Inorganic Chemistry · Solid State Crystal Defects and Non-Stoichiometry A perfect crystal would have zero entropy of disorder, which thermodynamics forbids above absolute zero. Defects are not flaws but a requirement. BSc & MSc · Inorganic Chemistry · Concept The short answer: Point defects exist in every real crystal because they increase entropy enough to lower the free energy despite costing enthalpy. Schottky defects remove ion pairs and reduce density; Frenkel defects relocate an ion and leave density unchanged. Non-stoichiometric compounds arise when defects are accompanied by a change in oxidation state. Why defects must exist Defects are thermodynamically required,…

X-Ray Diffraction and Bragg’s Law: Reading a Powder Pattern

September 4, 2026

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| Chemical Science Exams, Physical Chemistry

Solid State · Entrance Exams X-Ray Diffraction and Bragg’s Law: Reading a Powder Pattern From a row of peaks on a diffractogram to a lattice type and a unit cell edge — the calculation examiners actually set. BSc & MSc · Physical & Solid State Chemistry · Method The short answer: Bragg’s law relates the angle at which a crystal diffracts to the spacing between its planes. For a cubic crystal the plane spacing depends on the Miller indices in a simple way, so the ratio of sin²θ values across the peaks reveals the lattice type directly — and the…

Solid State for Entrance Exams: Close Packing, Voids and Band Theory

September 1, 2026

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| Chemical Science Exams, Inorganic Chemistry

Inorganic Chemistry · Solid State Solid State for Entrance Exams: Close Packing, Voids and Band Theory Solid state questions are almost always either a packing calculation or a conduction argument. Both are formula-driven once the structure is identified correctly. BSc & MSc · Inorganic Chemistry · Concept and numericals The short answer: Identify the lattice type first — simple cubic, body-centred or face-centred — because everything else follows from it: atoms per unit cell, coordination number, packing efficiency and the relationship between edge length and radius. Band theory then explains conduction as the gap between the filled and empty bands….