Question:
Which of the following has the highest melting point?
Which of the following has the highest melting point?
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Use Fajan's rule: Small highly charged cations increase covalent character, which lowers melting point.
More ionic character → higher melting point.
Updated On: Feb 15, 2026
- LiBr
- NaBr
- MgBr$_2$
- AlBr$_3$
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The Correct Option is B
Solution and Explanation
Step 1: Understand factors affecting melting point of ionic compounds.
Melting point mainly depends on lattice energy.
Higher lattice energy → stronger ionic bonding → higher melting point.
Lattice energy depends on:
• Charge on ions
• Size of ions (smaller size → stronger attraction)
Step 2: Compare the given compounds.
(A) LiBr → Li$^+$ is very small and highly polarizing. It increases covalent character (Fajan's rule), lowering melting point.
(B) NaBr → Na$^+$ is larger than Li$^+$ and less polarizing. Hence, bonding is more ionic and lattice energy remains high.
(C) MgBr$_2$ → Though Mg$^{2+}$ has higher charge, it strongly polarizes Br$^-$, increasing covalent character and reducing melting point.
(D) AlBr$_3$ → Highly covalent in nature and exists as dimer (Al$_2$Br$_6$), so it has low melting point.
Step 3: Conclusion.
Among the given options, NaBr is the most ionic and thus has the highest melting point.
Melting point mainly depends on lattice energy.
Higher lattice energy → stronger ionic bonding → higher melting point.
Lattice energy depends on:
• Charge on ions
• Size of ions (smaller size → stronger attraction)
Step 2: Compare the given compounds.
(A) LiBr → Li$^+$ is very small and highly polarizing. It increases covalent character (Fajan's rule), lowering melting point.
(B) NaBr → Na$^+$ is larger than Li$^+$ and less polarizing. Hence, bonding is more ionic and lattice energy remains high.
(C) MgBr$_2$ → Though Mg$^{2+}$ has higher charge, it strongly polarizes Br$^-$, increasing covalent character and reducing melting point.
(D) AlBr$_3$ → Highly covalent in nature and exists as dimer (Al$_2$Br$_6$), so it has low melting point.
Step 3: Conclusion.
Among the given options, NaBr is the most ionic and thus has the highest melting point.
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