1) NaCl has a higher melting point than KCl because sodium has a smaller atomic size than potassium. 2) The charge and ionic radii of ions determine the melting point of ionic compounds, with higher charge and smaller radii resulting in stronger attraction and higher melting points. 3) While LiCl should have the highest melting point based on charge and size, it exhibits more covalent character due to polarization, resulting in the order of melting points being NaCl > KCl > RbCl > LiCl.

1. Malik Xufyan
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Only Chemistry Discussion
Reasoning Question Series
Q:No.35: Why NaCl is More Ionic Than KCl ???
or Why NaCl has High M.P than KCl
Ans:
1. Postassium has larger atmic size than sodium
2. Due to small size of sodium combine with Chloride ion to form sodium chloride .
Comparison of melting points of ionic compounds is generally done by
considering the following two factors:
1. Charge of the cation/anion :
More the charge of cation or anion, stronger will be the forces of attraction
between the ions and higher will be the melting point.
2. Ionic radii:
More the distance between ions, lesser will be the strength of the bond giving rise
to lesser melting point.
Going by the above rules, the order should have been:
LiCl>NaCl>KCl>RbCl
(Since charges of the ions are same for each molecule and cationic radius increases down
the group.)
But LiCl, due to excessive polarization (Fajans' rules), exhibits high covalent character and
is placed last in the order. Hence, the new order would be:
NaCl>KCl>RbCl>LiCl
The other ions, being similar in size to chloride ions, do not undergo much polarization.