(a) Calculate the number of unit cell in 8.1g of aluminium if it crystallises in a fcc structure.
(Atomic mass of Al = 27`g*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.

Explanation

(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.