Class 12th

50. Option (i) $\sqrt[2]{2r}$ , $\frac{4r}{\surd 3}$  ,2r

In FCC crystal - the atoms touch each other at the face diagonals. If a is the edge length of the cube and r is the radius of the atom 

Therefore, $\sqrt[]{2a}=4r$

a = $\frac{4r}{\surd 3}$

In a simple cubic crystal - The atoms at the corners touch each other.

Therefore, a = 2r

49. Option (iv) 8 tetrahedral voids within the unit cells is correct since in ccp structure there are 4 atoms per unit cell and the number of tetrahedral voids is twice the number of atoms (i.e. eight tetrahedral voids per unit cell of cubic close packing.

48. Option (i) Frenkel defect is correct since in this defect an atom is displaced from its lattice point to an interstitial site thus creating a vacancy at the lattice point. This usually occurs in those ionic crystals where the size of anion is quite large as compared to that of the cation. Here dislocation of atoms occurs and hence it is known as dislocation defect.

47. Option (ii) The correct order of the packing efficiency is fcc > bcc > simple cubic

Packing Efficiency (fcc) - 74% 

Packing Efficiency (bcc) - 68% 

Packing Efficiency (simple cubic) - 52% 

46. Option (i) all the domains get oriented in the direction of magnetic field is correct since whenever a ferromagnetic substance is placed in a magnetic field it becomes a permanent magnet as all the domains get oriented in the directions of the magnetic field even after removal of  the applied magnetic field.

45. Option (iv) The fraction of octahedral or tetrahedral voids occupied depends upon the radii of the ions occupying the voids is not true since the occupation of the voids depends upon the stoichiometry of the compound instead of the radii of the ions. 

44. Option (ii) Ferromagnetic substances cannot be magnetised permanently is correct since ferromagnetic substances are those which are very strongly attracted by the magnetic field and can be magnetized permanently.

43. Option (ii) n-type semiconductor is correct since silicon is doped with electron rich impurities like Phosphorus or Arsenic and this leads to the increase in the conductivity due to the negatively charged electron . Hence, silicon doped with electron rich impurity is termed as n -type semiconductor. 

42. Option (iv) Electronic defects is correct since doping can be done with an impurity which is electron rich or electron deficient as compared to the intrinsic semiconductor like Si or Ge . Such impurities lead to the electronic defects in them. 

41. Option (iii) 4 is correct since in  2-D scope structure each sphere is in contact with four of its neighbours. Thus, its coordination number is found to be 4.