Structure of Some Ionic Compounds



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Tertahadral & Octahedral Voids, HCP & CCP

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Concepts of Voids, HCP & CCP Arrangements Density of unit cell- Tetrahedral and Octahedral void-  Tetrahedral Void-                                                                              A simple triangular void in a crystal which is surrounded by four spheres is called tetrahedral void. A tetrahedral void is developed when triangular voids have contact with one sphere either in the upper layer or in the lower layer. Tetrahedral void means the tetrahedral arrangement of spheres.                                            .     Characteristics of tetrahedral void- (i) Ratio of radii of void and constituent particle is constant.   r/R = 0.225 (ii) Co-ordination number  of particle present in the void is 4 (iii) Each constituent particle makes two tetrahedron with three spheres above and three spheres below (iv) No. of tetrahedral voids    = 2 ×No. of atoms    = 2× octahedral voids Relationship B/W  radius of void (r) and radius o
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Basic Concepts of Chemical Crystallography

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CHEMICAL CRYSTALLOGRAPHY- A branch of physical science that deals with the study of crystals i.e. their source, classification, properties and structure etc. is called chemical crystallography. A crystalline solid consist of a large number of small units called crystal. A crystal is a solid figure with a definite geometrical shape, flat faces and sharp edges. Crystal lattice and unit cell- “Regular arrangement of constituent particles i.e. atoms, ions or molecules of a crystal in a three dimensional space is called crystal lattice or space lattice.” “The smallest three dimensional portion of a complete space lattice which when repeated over and again in different directions to generate the complete space lattice is called the unit cell”       The situation may be compared with that of a thick wall made up of bricks, where brick is similar to the unit cell and the wall to the complete space lattice. The unit cell may be considered as the fundamenta
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Relation between nearest neighbour distance and radius of atom, Concept of Radius ratio

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Relationship between the nearest neighbour distance (d), edge of the unit cell (a) and the radius of the atom (r) (1)Simple cubic unit cell- Distance between nearest neighbour (d) = AB = a Hence for simple cubic (SC)  d = a   and r = a/2 (2)Face centred cubic unit cell-    Distance between nearest neighbour (d) =  AC (Because spheres at the face diagonal are touching each other)    But from  ∆ ABC      AC2 = AB2 + BC2                       Hence        d = a2 + a2                                  d =  2a2                             d =  √2a                             d =a/ √2   Since atomic radius (r = d/2)                                      r=  AC/2 = a/2√2                                                                    Hence for face centred cubic (FCC)  d =a√2         and   r =  a/2√2 (3) Body centred cubic unit cell- The distance between nearest neighbour (d) = 1/2 CD  Since all the spheres at the corners touch the central sphere, so CD
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