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.

Calculate the number of unit cells in 8.1g of aluminium if it crystall

1.	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.
Answer» SOLUTION :(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