(a) Calculate the number of unit cell in 8.1g of aluminium if it crystallises in a fcc structure. (Atomic mass of Al = 27g*mol^(-1)) (b) Give reasons: (i) In stoichiometric defects, NaCl exhibits Schottky defect and not Frenkel defect. (ii) Silicon on doping with Phosphorous forms n-type semiconductor. (iii) Ferrimagnetic substances show better magnetism than anti-ferromagnetic substances.

(a) Calculate the number of unit cell in 8.1g of aluminium if it cryst

1.	(a) Calculate the number of unit cell in 8.1g of aluminium if it crystallises in a fcc structure. (Atomic mass of Al = 27g*mol^(-1)) (b) Give reasons: (i) In stoichiometric defects, NaCl exhibits Schottky defect and not Frenkel defect. (ii) Silicon on doping with Phosphorous forms n-type semiconductor. (iii) Ferrimagnetic substances show better magnetism than anti-ferromagnetic substances.
Answer» Solution :(a) NUMBER of aluminium atoms present in 8.1 g of `Al=(8.1xx6.022xx10^(23))/27` In fcc structure, each unit cell consists of 4 Al atoms as Al crystallises in fcc structure. Therefore, number of unit cell in 8.1 g of Al `=(8.1xx6.022xx10^(23))/(27xx4)=4.5165xx10^(22)` (b) In ferrimagnetic substances, in PRESENCE of magnetic FIELD domains are aligned in opposite direction in UNEQUAL number and hence they have some NET magnetic momnet. On the other hand, in anti-ferromagnetic substances, in presence of magnetic field the domains are aligned in opposite direction in equal number. So they cancel magnetic moment completely and hence zero magnetic moment.