Question

Among the following, the one that has the lowest melting point is

• A. LiBr
• B. AlBr$_3$
• C. NaBr
• D. MgBr$_2$

Hint:

Compounds with more covalent character generally have lower melting points compared to ionic compounds

Answer 1

The Correct Option is B

Step 1: Understand melting point trend.
Melting point depends on the strength of intermolecular or interionic forcesIonic compounds generally have high melting points due to strong electrostatic forces

Step 2: Identify nature of given compounds.
LiBr, NaBr and MgBr$_2$ are predominantly ionic compoundsThus, they have strong lattice forces and high melting points

Step 3: Analyze AlBr$_3$.
AlBr$_3$ is largely covalent due to high polarization of Al$^{3+}$ ionIt exists as a dimer Al$_2$Br$_6$ in solid and liquid states

Step 4: Compare intermolecular forces.
Covalent compounds like AlBr$_3$ have weaker intermolecular forces compared to ionic lattice forces

Step 5: Effect on melting point.
Due to weaker intermolecular interactions, AlBr$_3$ melts at a much lower temperature than ionic bromides

Step 6: Compare with others.
LiBr, NaBr and MgBr$_2$ all have strong ionic bonding and hence higher melting points compared to AlBr$_3$

Step 7: Conclusion.
Thus, the compound with the lowest melting point is
\[ \boxed{\text{AlBr}_3} \]