The group IA alkali metals crystallize in the bcc structure whereas the group IB coinage metals crystallize in the fcc structure. This makes alkali metals salt, with low densities and low melting points because of low cohesive energies in comparison with the coinage metals. Among group II metals Be, Mg, Zn and Cd crystallizes in hcp ; Ca & Sr crystallizes in fcc and Ba crystallizes in bcc lattice. The crystal structure of transition metals is difficult to predict as many of these form allotropes having different crystal structures and physical properties. Thus iron has three allotropes, the α and δ allotropes have bcc while the γ allotrope has the fcc structure. Similarly for Cr, α allotrope is bcc but the β allotrope is hcp.
(i) In cubical lattice most dense plane is
DEL → Donor electron level AEL → Acceptor electron level Which of the following figure represent n-type conduction ? None of these (v) Which of the following figure represent p-type conduction for Si lattice referring to the figure given in (iv) ? None of these
Text Solution
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(i)
Sol. (1 1 1)
(ii)
Sol. In fcc the (1 1 1) plane contain largest number of atoms and the plane is perpendicular to the four cubic diagonals whereas in hcp lattice ( 1 0 0) plane is most dense and it is perpendicular to the one coordinate axis
(iii)
Sol. By absorbing the light falls on the metal surface, the electrons are excited. When these excited electrons return from the excited state to the ground state in one or more jumps, the light of all wavelengths in the visible region is emitted.
(iv)
Sol.
(v)
Sol.
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