26. A rhombohedral unit cell has a r = 5Å and a = 75°. Volume of the unit cell is:
(a) 120 Å
3
(b) 118 Å
3
(c) 161 Å
3
(d) 114 Å
3
27. A rhombohedral unit cell has a r = 5Å and a = 75°. Volume of the hexagonal
unit cell is:
(a) 340 Å
3
(b) 342 Å
3
(c) 344 Å
3
(d) 346 Å
3
28. In a close packing of identical spheres, the ratio of number of octahedral voids
around a sphere and the number of spheres around an octahedral void is:
(a) 1
(b) 2
(c) 3
(d) 4
29. In a close packing of identical spheres, the ratio of number of tetrahedral voids
around a sphere and the number of spheres a tetrahedral void is:
(a) 1
(b) 2
(c) 3
(d) 4
30. The critical radius ratio for a triangular coordination is:
(a) 0.155
(b) 0.225
(c) 0.414
(d) 0.732
31. The critical radius ratio for a tetrahedral coordination is:
(a) 0.155
(b) 0.225
(c) 0.414
(d) 0.732
32. The critical radius ratio for an octahedral coordination is:
(a) 0.155
(b) 0.225
(c) 0.414
(d) 0.732
1.5 Close Packing of Identical Atoms (Spheres)
39