The melting pt. of group 13 follows the order

The temperature at which both the solid and the liquid state of a pure substance exists is known as melting point. Down the group, the atomic size increases and the effective nuclear charge decreases. Therefore, it is observed that melting point of atomic lattice decreases down the group.

But in case of group-13 elements, there is an unusual trend of melting point which is:

B>Al>Tl>In>Ga

The unusual trend is observed due to structural properties of Ga metal. In Ga, the distance between one and another is around $2.43 \stackrel{\circ }{\mathrm{A}}.$ As a result, there is discrete arrangement of Ga atoms in Ga lattice which is responsible for the lowest melting point of Ga. Also, due to lanthanide compression, the atomic radius of Tl is lesser than In, therefore melting point of Tl is greater than In.