Among the Group IIIA elements, only aluminum is appreciably common in the Earth's crust (6%), and none of these elements is necessary to living organisms. Gallium, indium, and thallium are rare and relatively unimportant.

Gallium sometimes is used in thermometers because of its great liquid range; it melts at 30°C but boils only above 2500°C. The boiling point is normal in comparison with nearby elements in the table, and it is the melting point that is unusually low. This probably arises because gallium has an open, irregular crystal structure that is easily disarrayed.

The melting point of indium also is depressed, but not as much as gallium. Because gallium expands when it freezes (another indication of weak association of atoms in the solid) it finds a minor use in dental alloys and type metal. Both fillings and type must expand as they solidify to take a sharp impression of their moulds.

Indium is used in some alloys, and thallium salts find their main use in odorless, tasteless rat poisons.