Question

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

Answer 1

Melting and boiling point for alkali metals is directly proportional to its strength of metallic bond. And metallic bond strength is inversely proportional to the size of bonded metals.

Complete step-by-step answer: Alkali metals are bonded through weak interatomic bonds due to their large atomic radius and presence of only one valence electron. When we move down the group size of metal atom increases and so the repulsion between non-bonding electrons also increases and therefore melting and boiling point decrease down the group from lithium to Cesium. Normally all the metals are found in bonded states not in free state, so the metallic bonds can be used to explain the melting point of metals. Metals have low ionisation potential hence they can easily lose electrons which occupy the space between lattice and the left part is known as kernel which occupies the lattice sites. And the nature of metallic bonds can be explained by the attraction between these kernels and mobile electrons.
Hence the order of melting point and boiling point of alkali metals can be given as –
$Li > Na > K > Rb > Cs$
Hence in the above problem minimum melting point is of $Cs$ .
$Hg$ is not an alkali metal. It is a transition metal. Mercury is a metal but found in liquid state at room temperature as its melting point is very low.

Note: Melting point and boiling point for s-block molecules can be explained by the lattice energy and ionic characters respectively. Lattice energy can be understood as the ratio of charges to sum of radius of cation and anion.
$L.E. = \dfrac{{{q^ + }{q^ - }}}{{{r^ + } + {r^ - }}}$
Melting point is directly proportional to lattice energy of the molecule. For the same anion down the group melting point decreases as size of cation increases so lattice energy decreases.