77.
77. Correct order of melting point of Boron family.
(1) B < Al < Ga < In < Tl
(2) B > Al > Ga > In > Tl
(3) B > Al > Tl > In > Ga
(4) B > Tl > In > Al > Ga
The Correct order of melting point of Boron family is as follows:- B > Al > Tl > In > Ga (option 3)
The melting point (m.p) of Boron family do not show a regular trend . This is may be due to unusual crystal structure of Boron and gallium.
The melting point decreases very fastly on moving down the group from B to Ga and then increase from Ga to Tl. In the boron family, gallium has the lowest melting point and Boron has high melting point because its crystal structure consists of icosahedral units with boron atoms at all the 12 corners and each Boron atom is bonded to 5 equidistant neighbours. The crystal structure of gallium is quite different from that of boron. Each gallium metal atom has one close neighbour at a distance of 243pm and 6 more distant neighbours at a distance of and 279 pm270 pm. Ga consist of almost discrete diatomic molecule and hence its melting point is low.Al, In and Tl have closed packed structure. Their melting point decreases from Al to In and increases again for Tl.
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