Terbium · element 65
The rare earth that makes fluorescent lamps glow green and lets a coin sized alloy stretch on command
Key facts
A single Swedish quarry gave the periodic table four elements, and terbium is one of them. Ytterby, near Stockholm, produced a dark mineral that chemists spent most of the nineteenth century taking apart, and terbium was among the pieces. Its name is a slice of the village's name, as are yttrium, erbium and ytterbium.
Terbium is a soft, silvery metal that is malleable enough to cut with a knife. It is one of the less abundant lanthanides and among the more expensive, yet it turns up in surprisingly everyday places. The green in many fluorescent tubes and older colour screens comes from terbium ions, and an alloy of terbium, dysprosium and iron changes shape when a magnetic field is applied, a trick engineers use in sonar and precision actuators.
Where it is found
Terbium is present in the usual rare earth minerals, monazite, bastnäsite and xenotime, but only in small quantities, typically well under one per cent of the rare earth content. The richest practical sources today are ion adsorption clays in southern China, where weathering has concentrated the heavier lanthanides in a form that is unusually easy to leach out.
Because it is scarce and concentrated in few places, terbium is regularly listed among the elements most at risk of supply disruption. Efforts to find alternative sources include recycling fluorescent lamp waste and reprocessing old magnets.
How it was discovered
In 1843 the Swedish chemist Carl Gustaf Mosander showed that yttria, an oxide from Ytterby that had been treated as a single substance for fifty years, was actually three. He kept the name yttria for the colourless main fraction and named the other two erbia and terbia. Confusingly, later chemists swapped the two names around, so what Mosander called erbia is what we now call terbia.
Mosander was a student of Berzelius and worked in the same laboratory in Stockholm. His careful fractional precipitations also uncovered lanthanum and didymium. Pure terbium was not isolated until ion exchange methods arrived in the twentieth century.
What we use it for
Terbium(III) ions emit a clean green light when excited, and this green is one of the three colours mixed in compact fluorescent lamps and trichromatic tubes to produce white light. It served the same purpose in colour television and computer monitor phosphors before flat panels took over, and it remains useful in some LED and display technologies.
The alloy Terfenol-D, made from terbium, dysprosium and iron, shows the largest room temperature magnetostriction of any commercial material. Put it in a magnetic field and it lengthens measurably; remove the field and it returns. This makes it ideal for sonar transducers, precise positioning devices, fuel injectors and flat speakers that turn a table or window into a sound source.
Terbium also appears in solid state devices, in magneto-optical recording materials, and as a dopant in calcium fluoride and strontium molybdate for lasers. Terbium based inks and glasses that glow green under ultraviolet light are used in anti-counterfeiting.
Reading the numbers
Terbium melts at about 1356 °C and boils near 3123 °C, comfortably above the working temperature of most industrial furnaces and consistent with a mid series lanthanide. Its density of 8.23 grams per cubic centimetre is a little greater than iron, following the steady rise in density along the lanthanide row as the atoms shrink.
Its configuration [Xe] 4f9 6s2 has nine electrons in the 4f shell, two beyond the half filled point reached by gadolinium. It forms +3 ions in almost all its chemistry, but it can also lose a fourth electron to reach a half filled shell again, which is why terbium(IV) oxide exists when most lanthanides refuse to go beyond +3. A first ionisation energy of about 566 kJ/mol and electronegativity of 1.1 mark it as a typical electropositive rare earth. Its atomic mass of 158.93 is almost a whole number because natural terbium consists of a single isotope, terbium-159.
Safety and the environment
Terbium and its compounds are of low toxicity and have no known biological role. As with other rare earth metals, fine terbium powder can ignite and should be handled with care. Exposure is rare because the metal is used in small amounts inside sealed devices.
The main environmental issue is upstream. Leaching terbium from clay deposits has caused serious pollution in parts of southern China, with acid and ammonium salts contaminating soil and water. Recovering terbium from spent fluorescent lamps reduces both landfill mercury and the need for fresh mining.
Did you know?
- Terbium is one of four elements named after the village of Ytterby in Sweden, along with yttrium, erbium and ytterbium.
- The names terbia and erbia were swapped by later chemists, so the substance Mosander called erbia is the one we now call terbium oxide.
- Terfenol-D, an alloy of terbium, dysprosium and iron, physically stretches in a magnetic field and can turn a window or desk into a loudspeaker.
- Natural terbium is made of a single isotope, terbium-159, which is why its atomic mass is so close to a whole number.
- Unlike most lanthanides, terbium can form a +4 oxidation state, because losing a fourth electron leaves its 4f shell exactly half full.
More in the lanthanides
The nearest neighbours of terbium in the family, by atomic number.
See all 15 lanthanides on one page, or browse all 118 elements.
Frequently asked questions
What is terbium used for?
Terbium provides the green phosphor in fluorescent lamps and older colour screens, and it is a key component of Terfenol-D, a magnetostrictive alloy used in sonar, actuators and flat speakers. It is also used in some lasers, magneto-optical materials and security inks.
Why is terbium so expensive?
It is one of the scarcest rare earths, making up a tiny fraction of most ores, and its richest sources are concentrated in a few clay deposits in southern China. Separating it from chemically similar neighbours takes many processing stages, all of which adds cost.
Where does the name terbium come from?
It is named after Ytterby, a village near Stockholm whose quarry produced the mineral in which the rare earths were first found. Carl Gustaf Mosander coined the name in 1843 when he split the oxide yttria into three fractions.
Is terbium toxic?
Terbium compounds are considered to have low toxicity and the element has no known role in living things. The metal powder is flammable like other rare earths, but the main risks associated with terbium come from the environmental impact of mining rather than from the element itself.
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Written by Anthony. Updated September 2026. Numerical data from Periodic-Table-JSON (CC BY-SA), cross-checked against the Terbium reference article. Spotted an error? Tell us.