A. Very Short – Answer Questions
1. What is crystalline solid?
2. What is an amorphous solid?
3. Name any five amorphous solids.
4. Name any five crystalline solids.
5. What is anisotropy?
6. Why are crystals anisotropic?
7. Under which situations can an amorphous substance change to crystalline form?
8. What are different types of crystalline solids?
9. What are the characteristics of a molecular solid?
10. Why are liquids and gases categorised as fluids?
11. Why are solids incompressible?
12. What is coordination numbers?
13. What is a unit cell?
14. What is primitive lattices?
15. What are centred lattices?
16. An element A adopts a body-centred cubic structure. How many atom does the atom at the body-centre touch? What about the atom at the corner?
17. What is packing fraction?
18. A large number ,N, of circular discs of negligible thickness are arranged in a plane in
(i) square packing, and (ii)hexagonal packing
What is the total number of holes in (i) and (ii)? Calculate the packing fraction in each case.
(i) square packing, and (ii)hexagonal packing
What is the total number of holes in (i) and (ii)? Calculate the packing fraction in each case.
19. How many tetrahedral and octahedral holes are present in a FCC unit cell?
20. What is a point defect?
21. What is a Schottky defect?
22. What is Frenkel defact?
23. What is an F-centre?
24. Why does table salt, NaCl, sometimes appear yellow in colour?
25. Why is FeO (s) not formed in stoichiometric composition?
26. How do the energy band diagrams of metals, insulator and semiconductor differ?
27. How does temperature affect the conductivity of metals, insulators and semiconductors?
28. What is doping?
29. What element should Ge be doped with to get
(i) an n-type semiconductor, and (ii) a p-type semiconductor?
(i) an n-type semiconductor, and (ii) a p-type semiconductor?
30. Explain why does conductivity of germanium crystals increase on doping with galium.
B. Short-Answer Questions
1. How solids are different from liquids and gases?
2. Why does the rapid cooling of molten material result in formation of amorphous material?
3. Why is diamond hard and graphite soft, though both are network covalent solids?
4. Graphite is expected to conduct electricity in directions parallel to its sheet, but it conducts in all directions. Why does this happen?
5. Ionic compounds do not conduct electricity in the solid state but do so on melting or in aqueous solution. Explain.
6. Which type of solid has highest melting point?
7. With reference to 3-d lattices, how many unit cells are the following points common to?
(i)Corner (ii)Face-centre
(iii)Body-centre (iv)Edge-centre
8. With reference to a cubic unit of cell of edge-length a, what are the distances between
(i) The corner points on a face diagonal,
(ii) The corner points on a body diagonal,
(iii) The face-centre points on perpendicular faces,
(iv) The face-centre points on parallel faces,
(v) A corner point and the nearest face-centre point,and
(vi) A corner point and the body centre?
9. What is the maximum separation between the lattice points in a (i) primitive cubic unit cell, (ii)body-centred cubic unit cell, and (iii) face-centred cubic unit cell?
10. What are HCP and CCP? Which of these has a higher packing of spheres?
11. What are tetrahedral and octahedral holes in the closest packing of spheres?
12. Why does the structure of CsCI differ from that of NaCI?
13. Why defects are present in crystal structures at ordinary temperatures and why do they increase with rise in temperature?
14. Why is a cation Frenkel defect more common than an anion Frenkel defect?
15. Anionic Frenkel defect is found in ionic solids with a fluorite structure. Why is this so?
16. Why does ZnO turn yellow on heating and become white again on cooling?
17. What is a nonstoichiometric defect? Give an example.
18. What are valence and conduction bands?
19. Explain the electrical conductivity of metals in terms of the band theory.
20. Semiconductor are insulator at 0 K. How do they become weakly conducting as temperature increases?
21. What is the difference between intrinsic and extrinsic semiconductors?
22. What are n-type and p-type semiconductors?
23. What are the different types of magnetic substances?
24. How does a diamagnetic substance differ from a paramagnetic substance?
25. Describe the different types of magnetic behavior shown by solids.
C. NUMERICALS
1. NH4Cl crystallizes in a body centred cubic lattice, with a unit cell distance of 387 pm. Calculate (a) the distance between the oppositely charged ions in the lattice
(b) the radius of the NH4+ ion if the radius of the Cl- ion is 181 pm.
2. Copper has the fcc crystal structure. Assuming an atomic radius of 130pm for copper atom.
(Cu = 63.54):
(a) What is the length of the unit cell of Cu?
(b) What is the volume of the unit cell?
(c) How many atoms belong to the unit cell?
(d) Find the density of Cu.
3. The density of CaO is 3.35 gm/cm3. The oxide crystallises in one of the cubic systems with an edge length of 4.80 Å. How many Ca++ ions and O–2 ions belong to each unit cell, and which type of cubic system is present?
4. A metal crystallizes into two cubic system-face centred cubic (fcc) and body centred cubic (bcc) whose unit cell lengths are 3.5 and 3.0Å respectively. Calculate the ratio of densities of fcc and bcc.
5. Non-stoichiometric cuprous oxide, Cu2O can be prepared in the laboratory. In this oxide, copper to oxygen ratio is slightly less than 2:1 can you account of the fact that this substance is a p-type semiconductor
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