Discussion
Among the following, the one that has the lowest melting point is
Answer: B
The lowest melting point belongs to AlBr3.
Aluminium bromide melts at about 98 ℃, while LiBr, NaBr and MgBr2 all melt above 550 ℃. The reason is the nature of the bonding. Al3+ is very small and highly charged, so it strongly polarises the large, soft bromide ion (Fajans' rules). The bonding becomes largely covalent, and the solid is built from discrete Al2Br6 molecules held together only by weak intermolecular forces.
LiBr, NaBr and MgBr2 are ionic lattices, in which melting means overcoming strong electrostatic attraction throughout the crystal. LiBr and MgBr2 have some covalent character from their small cations, but far less than AlBr3, so their melting points stay high.
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