39. Which of the following statement is not true about the hexagonal close packing? 

(i) The coordination number is 12. 

(ii) It has 74% packing efficiency. 

(iii) Tetrahedral voids of the second layer are covered by the spheres of the third layer. 

(iv) In this arrangement spheres of the fourth layer are exactly aligned with those of the first layer. 

· Density of solid decreases in Schottky defect and vacancy defect.

· Density of solid increases in interstitial defect.

· Density of solid remain unchanged in Frenkel defect.

77. Option (iii) Assertion is correct statement, but reason is wrong statement is the answer since intermediate conductivity in semiconductor is due to the small energy gap between valence band and conduction band and hence the assertion is correct, but the reason is wrong.

76. Option (ii) Assertion and reason both are correct statements, but reason is not correct explanation for assertion is the answer since in fcc structure the number of atoms present = 4 per unit cell which provides a maximum packing efficiency of 74%. 

75. Option (iii) Assertion is the correct statement, but the reason is wrong is the answer since besides the body centre there is one octahedral void at the centre of each 12 edges which is shared between four adjacent unit cells. Thus, Octahedral voids present at the body centre of the cube = 1 

12 octahedral voids are located at each edge and shared between four-unit cells = 12 * $\frac{1}{2}$  = 3

Total number of octahedral voids = 4.