Question

Which of the following has the highest melting point?
(A) Boron
(B) Aluminium
(C) Gallium
(D) Indium

Answer 1

Boron, aluminium, gallium and indium are elements of the same group that is group 13. Group 13 is also known as the Boron family. Boron is a metalloid whereas aluminium, Gallium, Indium are metals.

Complete step by step solution:
The melting point is the temperature at which the substance in solid-state changes to the liquid state. The melting point depends on the electropositivity of an element, which in turn depends on the ionization energy of the element. In this case, the metallic character of elements decides the melting point. Boron, aluminium, gallium and indium belong to group 13. Down the group electropositivity decreases but, there is a slight deviation from indium to thallium. An important and interesting factor in group 13 is that all the elements except boron are metals and boron is a semimetal. Semimetals are the elements that acquire the characteristics of both metals and nonmetals. As electropositivity increases down the group, the melting point decreases down the group steadily till gallium but from indium to thallium there is an increase in melting point.
From this, we can come to the conclusion that aluminium and gallium have a low melting point. Now we have to decide between boron and indium. Since boron is semi-metal whereas indium is a metal and boron forms the icosahedral structure. The icosahedral structure of boron consists of a rhombohedral unit cell with 12 atoms. Generally, an increase in the metallic character of the element, the boiling and melting point will decrease in the same direction. So boron has the highest melting point. Another factor that attributes for the highest melting point of boron is its icosahedral structure. Breaking bonds in an icosahedral structure takes time and energy as this structure is bit complex in comparison to a crystal that has a number of a cubic unit cell.

Thus, option (A) is the correct answer.

Note: Metallic character mainly attributes to the melting point of elements. Electropositivity means nothing but a metallic character. The Icosahedral structure of boron is the main factor that attributes more for its highest melting point.