(a) Calculate the number of unit cells in 8.1g of aluminium if it crystallizes in a f.c.c., structure. (Atomic mass of AI `= 27 g mol^(-1)`)
(b) Given reasons:
(i) In stoichiometric defects, NaCl exhibits Schottky defect and not Frenkel defect.
(ii) Silicon on droping with Phosphorus forms n-type semiconductor.
(iii) Ferrimagnetic substances show better magnetism than antiferromagnetic substances.

Explanation

(a) 27g of aluminium have `6.02 xx 10^(23)` atoms of Al
8.1 g of aluminium have `(6.02 xx 10^(23))/(27) xx 8.1`
`= 12.06 xx 10^(22)` atoms of aluminium
Now in FCC structure :
1 unit cell has 4 aluminium atoms i.e.,
`12.06 xx 10^(23)` atoms have `(1)/(4) xx 12.06 xx 10^(22)` unit cells `= 3.05 xx 10^(22)` unit cells
(b) (i) NaCl exhibits schottky defect because of the small difference in the size of cations and anions as i Frenkel defect, large difference in the size of cations and anions.
(ii) Phosphorus have `5e^(-)s` in the valence shell, hence, it forms four covalent bonds with silicon atoms, the fifth extra `e^(-)` is free and gets delocalized and increase the conductivity of silicon. Hence, because of the presence of free `e^(-)`, it forms n-types semiconductor
(iii) Ferrimagnetic substances possess small net magnetic moment but antiferromagnetic substances posses zero net magnetic moment because it contains equal no. of `e^(-)s` with opposite spin but ferrimagnetic have unequal no. of `e^(-)s` with opposite spin