Question

Which metal from the following alkali metals has the lowest melting point?
A) $Hg$
B) $K$
C) $Rb$
D) $Cs$

Answer 1

As we know that according to Fajan’s rule, the element with maximum covalent character will have a low melting point. The larger atomic size attributes to the low melting point and boiling point and moving down the group, atomic size increases and strength of metallic character decreases.

Complete step by step answer:
As we know that alkali metals have a low melting point and boiling point. The low melting points are attributed to their larger atomic size, due to which the binding energies of their atoms in the crystal lattice are low.
Further, we also know that on moving down the group, the atomic size of alkali metals increases and the strength of metallic bonds decreases which causes the decrease in their melting points.
As we know that Fajan’s rule states that anion having the greater size will get easily polarised and thus will have a minimum covalent character and a cation having smaller size will be having higher covalent characters.
Therefore, Cesium possess the lowest melting point which is about $28.5^\circ C$ but among the metallic elements, mercury possess the lower melting point than that of cesium which is approximately $38.9^\circ C$ this is due to the filled outer electrons shell and contraction of outer orbital which is closer to the nucleus.
Therefore, the correct answer is (A).

Note: Remember that if the covalent character is maximum, then the melting point will be the lowest for the particular compound because the ionisation enthalpy is low and as the covalent character increases, ionic character of that compound decreases. The boiling point of alkali metals follows the same order as the melting point that melting point decreases down the group.