JEE Main 2021 · 27 July · morning · Single correct · +4 / −1
Given below are two statements : One is labelled as Assertion A and the other labelled as Reason R.
Assertion A : Lithium halides are some what covalent in nature.
Reason R : Lithium posses high polarisation capability.
In the light of the above statements, choose the most appropriate answer from the options given below :
- (a) A is true but R is false
- (b) A is false but R is true
- (c) Both A and R are true but R is NOT the correct explanation of A
- (d) Both A and R are true and R is the correct explanation of A
Answer and solution
Answer (d) Both A and R are true and R is the correct explanation of A
Solution
Chemistry ma’am · 4 steps
A tiny Li⁺ pulls the halide's electron cloud towards itself, and that sharing is covalent character.
- Check R. Li⁺ is the smallest alkali metal ion, so its charge is packed into a very small volume. This high charge density gives it the highest polarising power in group 1, so R is true.
- Check A. A strongly polarising cation distorts the electron cloud of the anion next to it. The electron density gets partly shared between the two ions, so the bond is no longer purely ionic. This is why LiCl, LiBr and LiI dissolve in organic solvents such as ethanol, and why LiCl melts lower than NaCl. A is true.
- Link them. By Fajans' rules, a small, highly polarising cation is exactly what gives an ionic compound covalent character. So the polarising power stated in R is the direct cause of A.
- So. Both statements are true and R correctly explains A. The answer is (d).
The idea to keep. Small cation, high charge density, high polarising power, more covalent character. This one Fajans' chain explains most of the anomalous behaviour of Li and Be.
Answer from the NTA final answer key. Solution by PrepWiser.