Terbium is a member of the lanthanide series of elements, which stretches from lanthanum (La) through lutetium (Lu).
Although it is one of the least abundant lanthanides in the earth's crust, it was discovered relatively early - in 1843. Mosander, the discoverer of other elements (Y, La, and Er), decided to try to again study the oxide yttria. This oxide is a mixture of several others: ceria (cerium oxide); lanthana (lanthanum oxide); and didymia (a mixture of the oxides of praseodymium and neodymium). Mosander separated these from the yttria, and, in the portion that remained, he discovered two new oxides: a yellow oxide he called erbia and a rose-colored oxide he named terbia. From erbia he found the new element erbium, and the new element terbium came from terbia. In a single experiment Mosander found two as yet undiscovered elements.
The metal is soft and silvery. It has a higher density (8.23 g/cc) than any of the preceding lanthanides. In the solid state, the metal has a hexagonal closest packed structure.
Like many of the lanthanide elements, terbium is obtained from the mineral monazite, a mixture of the phosphates of Ca, Th, Ce, and most of the other lanthanides.
The metal is obtained by reducing a halide with an "active" metal such as Ca or Li.
3 Li(s) + TbCl3(s) Tb(s) + 3 LiCl(s)
The reaction is carried out in a tantalum crucible at high temperature in a helium atmosphere (the latter being used to prevent oxidation of the metal).
The metal is also reactive, forming Tb(III) salts on reaction with acids.
The element is used in steel alloys and, as the oxide, in fluorescent bulbs.