Question

Which of the alkali metals is having least melting point?
A. \[Na\]
B. \[K\]
C. \[Rb\]
D. \[Cs\]

Answer 1

Hint: Melting point means the temperature at which a solid starts to melt.
Melting point of any element or molecule depends on the interatomic forces or the intermolecular attractive forces, respectively.
Alkali metals are s-block elements as they have one electron in their outer-most s-orbital.
We will solve this problem considering the concept of interatomic forces present in the given metals.

Complete step by step answer:
If we try to understand the positions of the given alkali metal atoms, we can easily see that all the given elements occupy the \[{1^{st}}group\] of the periodic table, they just vary by the period number.
\[Na{\text{ }}is{\text{ }}a{\text{ }}{3^{rd}}period{\text{ }}element.\]
\[K{\text{ }}is{\text{ }}a{\text{ }}{4^{th}}period{\text{ }}element.\]
\[Rb{\text{ }}is{\text{ }}a{\text{ }}{5^{th}}period{\text{ }}element.\]
\[Cs{\text{ }}is{\text{ }}a{\text{ }}{6^{th}}period{\text{ }}element.\]
Now, as we go downwards along a group, the atomic size increases. This means that the atomic size of Cesium \[\left( {Cs} \right)\] is the highest and the atomic size of Sodium \[\left( {Na} \right)\] is the lowest.
Now, because of that very large size of Cesium, it has very weak interatomic interactions and as a result the metallic bond between two Cesium atoms becomes very weak. So, cesium melts at lower temperature compared to sodium or potassium or rubidium.
If we see the literature values of melting points, we can see-

Elements	Melting Point (\[^0C\])
Sodium	98
Potassium	63
Rubidium	39
Cesium	28

So, Cesium possesses the lowest melting point among the given metals.So, option-(D) is the correct option.

Note:
Melting point and boiling point sound similar but they are conceptually different.
For a given group, melting point normally decreases downwards.
For a given group, boiling point normally increases downwards.