The ninth member of the lanthanide series, terbium is a fairly electropositive metal that reacts with water, evolving hydrogen gas.

Terbium is never found in nature as a free element, but it is contained in many minerals, including cerite, gadolinite, monazite, xenotime and euxenite.

Terbium is a silvery-white rare earth metal that is malleable, ductile and soft enough to be cut with a knife.

Only three electrons can be removed before the nuclear charge becomes too great to allow further ionization, but in the case of terbium, the stability of the half-filled [Xe]4f7 configuration allows further ionization of a fourth electron in the presence of very strong oxidizing agents such as fluorine gas.

[16] In solution, terbium typically forms trivalent species, but can be oxidized to the tetravalent state with ozone in highly basic aqueous conditions.

In aqueous conditions, terbium can be coordinated by nine water molecules, which are arranged in a tricapped trigonal prismatic molecular geometry.

Terbium combines with nitrogen, carbon, sulfur, phosphorus, boron, selenium, silicon and arsenic at elevated temperatures, forming various binary compounds such as TbH2, TbH3, TbB2, Tb2S3, TbSe, TbTe and TbN.

[33] It can be obtained by reacting terbium(III) chloride or terbium(III) fluoride with fluorine gas at 320 °C:[34] When TbF4 and caesium fluoride (CsF) is mixed in a stoichiometric ratio in a fluorine gas atmosphere, caesium pentafluoroterbate (CsTbF5) is obtained.

[35] The compound barium hexafluoroterbate (BaTbF6), an orthorhombic crystal with space group Cmma, can be prepared in a similar method.

It is slightly hygroscopic[39] and is the main terbium compound found in rare earth-containing minerals and clays.

[40] Other compounds include: Naturally occurring terbium is composed of its only stable isotope, terbium-159; the element is thus mononuclidic and monoisotopic.

[9] Terbium-149, with a half-life of 4.1 hours, is a promising candidate in targeted alpha therapy and positron emission tomography.

"Erbia", the oxide containing what is now known as terbium, originally was the fraction that was yellow or dark orange in solution.

[57] Terbium (as the species Tb II) has been detected in the atmosphere of KELT-9b, a hot-Jupiter planet outside the Solar system.

[58] Currently, the richest commercial sources of terbium are the ion-adsorption clays of southern China;[40] the concentrates with about two-thirds yttrium oxide by weight have about 1% terbia.

[59] Crushed terbium-containing minerals are treated with hot concentrated sulfuric acid to produce water-soluble sulfates of rare earths.

Calcium and tantalum impurities can be removed by vacuum remelting, distillation, amalgam formation or zone melting.

As a component of Terfenol-D, terbium is used in actuators, in naval sonar systems, sensors, and other magnetomechanical devices.

[66] Terbium oxides are used in green phosphors in fluorescent lamps, color TV tubes,[10] and flat screen monitors.

[10] In 2023, terbium compounds were used to create a lattice with a single iron atom that was then examined by synchrotron x-ray beam.

[69] Terbium, along with many of the other rare earth elements, is poorly studied in terms of its toxicology and environmental impacts.

[70] No values are established in the United States by the Occupational Safety and Health Administration or American Conference of Governmental Industrial Hygienists at which terbium exposure becomes hazardous, and it is not considered a hazardous substance under the Globally Harmonized System of Classification and Labelling of Chemicals.

[74][75] High exposures of terbium may enhance the toxicity of other substances causing endocytosis in plant cells.