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The Solid State

Short Answer Type

11. Name the parameters that characterize a unit cell.

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12. Distinguish between Hexagonal and monoclinic unit cells.

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13. Distinguish between Face-centred and end-centred unit cells.

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14. Explain how much portion of an atom located at (i) corner and (ii) body-centre of a cubic unit cell is part of its neighbouring unit cell.

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15. What is the two-dimensional coordination number of a molecule in square close packed layer ?

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16. A compound forms hexagonal closed-packed structure. What is the total number of voids in 0.5 mol of it? How many of these are tetrahedral voids?

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17. A compound is formed by two elements M and N. The element N forms ccp and atoms of M occupy 1/3 of tetrahedral voids. What is the formula of the compound?

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Multiple Choice Questions

18. Which of the following lattices has the highest packing efficiency:

simple cubic
body-centred cubic
hexagonal close packed lattice

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Short Answer Type

19. An element with molar mass 2.7 x 10^–2kg mol^–1 form a cubic unit cell with edge length 405 pm. If its density is 2.7 x 10³ kg m^–3. What is the nature of the cubic unit cell?

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20. What type of defect can arise when a solid is heated? Which physical property is affected by it and in what way?

Non-stoichiometric defects can arise when a solid is heated.
The crystals with this defect becomes coloured. Alkali halides like NaCl and KCl show this type of defect. When crystals of NaCl are heated in an atmosphere of sodium vapour, the sodium atoms are deposited on the surface of the crystal. The CI^– ions diffuse to the surface of the crystal and combine with Na atoms to give NaCl. This happens by loss of electron by sodium atoms to form Na⁺ ions. The released electrons diffuse into the crystal and occupy an ionic sites. As a result the crystal now has an excess of sodium. They impart yellow colour to the crystals of NaCl.

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