Question

Which of the following has the lowest melting point?
$A.\,Li$
$B.\,Na$
$C.\,K$
$D.\,Cs$

Answer 1

The melting point which is also denoted as M.P. Due to the large atomic size of alkali metals, the binding energies of their atoms in the crystal lattice is low. These alkali metals are having a low melting point. The melting point decreases down to the group. Therefore, the melting point decreases from Lithium ($Li$) to Rubidium ($Rb$).

Complete step by step answer:
- The temperature at which the solid starts melting is called the melting point. This M.P depends upon the intermolecular attractive forces between the molecules or the interatomic forces between the elements. Alkali metals belong to the s- block elements because their outermost electron shell configuration ends with one electron, i.e., outermost s orbital configuration is filled with one electron.
- As we know that alkali metals are the first group elements in the periodic table but just, they differ in the periods from second, third, fourth, fifth respectively. As we descend to the group, the size increases since the shells are increasing according to the configuration. So, the lithium is having the smallest size from the group and caesium is having a large size.
- Therefore, the caesium is having very weak interatomic interactions and results in the metallic bond between two caesium atoms becoming very weak. Hence, the caesium melts at the lower temperature as compared to the other elements except for rubidium. Rubidium is the metal that melts even at a lower temperature than caesium.
- From this, we are able to state that as we move down to the group the melting point decreases.

Hence the correct answer is option $D.\,Cs$.

Note: The alkali metals are shiny when they are cut freshly. These metals are having a lower melting point than the other group elements. The melting point decreases downwards. These metals react vigorously with water and produce hydrogen gases. As we go down to the group, the reaction becomes more vigorous with water.