Boron is non-metallic in nature. It is extremely hard and black coloured solid.

It exists in many allotropic forms. Due to very strong crystalline lattice, boron has unusually high melting point.

Rest of the members are soft metals with low melting point and high electrical conductivity. It is worthwhile to note that gallium with unusually low melting point ( $303 \mathrm{~K}$ ), could exist in liquid state during summer.

Its high boiling point ($2676 K$) makes it a useful material for measuring high temperatures.

Density of the elements increases down the group from boron to thallium.