Draw a neat labelled diagram of the stoichiometric defect observed in ionic solids having ions of similar size. What is the effect of this defect on the density of the crystal?

Stoichiometric defects are intrinsic defects in which the ratio of cations to anions remains exactly the same as represented by the molecular formula. Stoichiometric defects are also called intrinsic defects because they are due to the deviation from regular arrangement of atoms or ions in a crystal system and no external substance is being added to the system. They are mainly of two types: Vacancy defects: These are the defects in which an atom is not present at its lattice sites causing that lattice site to be vacant and create a vacancy defect. Due to this defect, the density of a substance will decrease. Interstitial defect: It is a defect in which an atom or molecule occupies the intermolecular spaces or interstitial sites in crystals. Because of this defect the density of the substance increases.

Write any two characteristics of molecular solids.

Molecular solids are defined as the compounds that are made up of atoms or molecules held together by London dispersion forces, dipole-dipole forces, or hydrogen bonds. Sucrose and solid carbon dioxide are the examples of molecular solids. Characteristics of molecular solids are: (i) They have low melting points. (ii) These are poor conductors of electricity.

Draw a neat labelled diagram to show Frankel defect and write any one condition for an ionic solid to show this defect.

Frenkel defect: When an ion is missing from the crystal lattice sites and occupies an interstitial site, electrical neutrality, as well as stoichiometry of the compounds, are maintained. This type of defect is called a Frenkel defect. Since cations are usually smaller, it is more common to find the cations occupying interstitial sites.This type of defect is present in ionic compounds which have (i) Low coordination number (ii) The larger difference in the size of cation and anions (iii) Compounds having highly polarising cation and easily polarisable anion. A few examples of ionic compounds exhibiting this defect are AgCl , AgBr , AgI , ZnS .

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The number of tetrahedral voids present per atom in a cubic close packed structure is _____ .

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