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The adjective, 'crystalline' when applied to solids, implies an ideal crystal in which the structural units, termed as unit cells, are repeated regularly and indefinitely in three dimensions in space. The unit cell, containing at least one molecule, has a definite orientation and shape defined by the translational vectors, a, band c. The unit cell therefore has a definite volume, V that contains the atoms and molecules necessary for generating the crystal. Every crystal can be classified as a member of one of the seven possible crystal systems or crystal classes that are defined by the relationships between the individual dimensions, a, band c of the unit cell and between the individual angles, \(\alpha, \beta \ and \ \gamma\) of the unit cell. The structure of the given crystal may be assigned to one of the 7 crystal systems, to one of the 14 Bravais lattices, and to one of the 230 space groups. These uniquely define the possible ways of arranging atoms in a three-dimensional solid. Based on these observations, seven crystal systems were identified: triclinic, monoclinic, trigonal or rhombohedral, tetragonal, hexagonal, rhombic or orthorhombic and cubic.
The following questions are multiple choice questions. Choose the most appropriate answer :
(i) The crystal system of a compound with unit cell dimensions, a = 0.387 nm, b = 0.387 nm and c = 0.504 nm and \(\alpha, \beta \ =90^o\) and \(\gamma\) = 120° is

(ii) The unit cell with the structure given below represents __________crystal system.

(iii) In a triclinic crystal

(iv) The unit cell with dimensions \(\alpha=\beta=\gamma=90^{\circ}, a=b \neq c \) is

Read the passage given below and answer the following questions:
In contrast to the disorders of gases and liquids, there is translational order in crystals. However, disordered or amorphous solids also exist which lack such order, they are really highly viscous liquids. In translational order entire structure or lattice, can be generated by repeated replication of a small regular figure, termed as unit cell. The planes of any crystalline structure can be specified using Miller indices, which is also serve to identify single crystal faces.
The ordered structure or lattice, of a solid can be determined by X-ray or neutron diffraction studies, in which a beam of X-rays of neutrons is scattered from the sample to produce a diffraction pattern which can be analyzed to reveal the crystal structure of the sample. All crystal lattices can be classified into 14 Bravais lattices belonging to 7 systems. For example, the simple cubic, face-centred cubic and body-centred cubic lattices are the 3 lattices of the cubic system. Cubic and hexagonal close- packed structures have the structure of tightly packed spheres where each sphere touches 12 neighbours, 6 in the same plane and 3 above and 3 below. These two close-packed structures differ in the placement of successive planes or layers. For the hexagonal close packing, a third layer is laid down to reproduce the first layer, so that the structure could be represented by ABABAB .... For cubic close packing, third layer is again displaced, corn spending to ABCABC ...
The following questions are multiple choice questions. Choose the most a propriate answer:
(i) In hexagonal close packing, a sphere has coordination number of

(ii) Which of the following arrangements correctly represents hexagonal and cubic close packed structure respectively?

(iii) The arrangement of the first two layers, one above the other in hep and eep arrangements is

(iv) Which of the following statements about amorphous solids is incorrect?

Read the passage given below and answer the following questions:
In ideally ionic structures, the coordination numbers of the ions are determined by electrostatic considerations. Cations surround themselves with as many anions as possible and vice versa. This maximizes the attractions between neighbouring ions of opposite charge and hence maximizes the lattice energy of the crystal. This requirement led to the formulation of the radius ratio rule for ionic structures in which the ions and the structure adopted for a particular compound depend on the relative sizes of the ions. Thus, for the stable ionic crystalline structures, there is definite radius ratio limit for a cation to fit perfectly in the lattice of anions, called radius ratio rule. This depends upon the ratio of radii of two types of ions, r₊/r_-.
This ratio for coordination numbers 3, 4, 6 and 8 are respectively 0.155 - 0.225, 0.225 - 0.414, 0.414 - 0.732 and 0.732 - 1.000. The coordination number of ionic solids also depends upon temperature and pressure. On applying high pressure, coordination number increases. On the other hand, on applying high temperature, it decreases.
The following questions are multiple choice questions. Choose the most appropriate answer:
(i) The ionic radii of K⁺, Rb⁺ and Br^- are 137, 148 and 195 pm. The coordination number of cation in RbBr and KBr structures are respectively

(ii) If the radius of Na⁺ ion is 95 pm and that of Cl^- ion is 181 pm, the coordination number of Na⁺ ion is

(iii) Which is not the correct statement for ionic solids in which positive and negative ions are held by strong electrostatic attractive forces?

(iv) If the pressure of CsCl is increased, then its coordination number will

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In an ideal crystal, there must be regular repeating arrangement of the constituting particles and its entropy must be zero at absolute zero temperature. However, it is impossible to obtain an ideal crystal and it suffers from certain defects called imperfections. In pure crystal, these defects arises either due to disorder or dislocation of the constituting particles from their normal positions or due to the movement of the particles even at absolute zero temperature. Such defects increase with rise in temperature. In addition to this, certain defects arise due to the presence of some impurities. Such defects not only modify the existing properties of the crystalline solids but also impart certain new characteristics to them.
The following questions are multiple choice questions. Choose the most appropriate answer:
(i) AgCI is crystallized from molten AgCI containing a little CdCI₂. The solid obtained will have

(ii)The ionic substances in which the cation and anion are of almost similar size shows

(iii) If A1³⁺ions replace Na⁺ ions at the edge centres of NaCl lattice, then the number of vacancies in 1mole of NaCI will be

(iv) Which of the following gives both Frenkel and Schottky defect?

Read the passage given below and answer the following questions:
The idealized ionic solid consists of two interpenetrating lattices of oppositely-charged point charges that are held in place by a balance of coulombic force of long range. But real ions occupy space, no such "perfect" ionic solid exists in nature. Chemists usually apply the term "ionic solid" to binary compounds of the metallic elements of groups 1 - 2 with one of the halogen elements or oxygen. The most well known ionic solid is sodium chloride, also known by its geological names as rock-salt or halite. Structurally, each ion in sodium chloride is surrounded and held in tension by six neighbouring ions of opposite charge; this is known as (6, 6) coordination. The resulting crystal lattice is of a type known as simple cubic. There are many other fundamental ionic structures (not all cubic) and these are:
Zinc blende structure (ZnS) : having ccp arrangement of S^2- and Zn²⁺ in alternate tetrahedral voids; Wurtzite structure (ZnS) : having hcp arrangement of S^2- and Zn²⁺ in alternate tetrahedral voids; Fluorite structure (CaF₂) : having ccp arrangement of Ca²⁺ and F^- in all tetrahedral voids; Antifluorite structure (Na₂O): having ccp arrangement of O^2- and Na⁺ in all tetrahedral voids. These solids tend to be quite hard and have high melting points.
The following questions are multiple choice questions. Choose the most appropriate answer:
(i) In NaCl crystal, each Cl^- ion is surrounded by

(ii) In an antifluorite structure, cations occupy

(iii) Antifluorite structure is derived from fluorite structure by

(iv) Ionic solid BaF₂ has which kind of structure?