Physical Properties

Physical Properties of Group 13

Boron stands apart from the rest of the group. It is a non-metal — an extremely hard, black solid that exists in several allotropic forms, held together by a very strong covalent crystal lattice. That strong lattice is why boron has an unusually high melting point for a p-block element.

Aluminium, gallium, indium and thallium, by contrast, are all soft metals: low melting points and high electrical conductivity, the hallmark of metallic bonding. Gallium is the most striking case — its melting point is only 303 K (just above room temperature), so a sample of gallium metal will melt in your hand or on a warm summer day, yet its boiling point is a very high 2676 K. This unusually wide liquid range is exploited to make gallium-based high-temperature thermometers. Density increases steadily down the group from boron to thallium.

Table 11.2 — Atomic and Physical Properties of Group 13 Elements (a = metallic radius, b = 6-coordinate ionic radius, c = Pauling scale)