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
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.