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.