Terbium
Terbium is a chemical element with symbol Tb and atomic number 65. It is a silvery-white rare earth metal that is malleable, ductile and soft enough to be cut with a knife. Terbium is never found in nature as a free element, but it is contained in many minerals, including cerite, gadolinite, monazite, xenotime and euxenite.
Identity
- Atomic mass
- 158.925 u
- Category
- Lanthanides
- Group
- 3
- Period
- 6
- Block
- f
- Oxidation States
- +3, +4
- Phase (25 °C)
- Solid
- Appearance
- silvery white
Electronic Structure
- Electron configuration
- 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f9
- Short configuration
- [Xe] 4f9 6s2
- Electrons per shell
- 2, 8, 18, 27, 8, 2
- Electronegativity (Pauling)
- 1.10
- Electron affinity
- 112 kJ/mol
- 1st ionization energy
- 566 kJ/mol
- Ionization energies
- 566 · 1110 · 2114 · 3839 kJ/mol
Physical properties
- Melting point
- 1355.8 °C (1629 K)
- Boiling point
- 3122.8 °C (3396 K)
- Density
- 8.23 g/cm³
- Molar heat capacity
- 28.9 J/(mol·K)
Chemical behavior of terbium
Terbium has nine outer electrons ([Xe] 4f⁹ 6s²) and acts as +3, with a rare +4 in the oxide TbO₂ and the fluoride TbF₄. Its electronegativity is very low (1.10) and it forms highly ionic bonds. It is soft, malleable and oxidizes readily. Its Tb³⁺ ions emit the purest green light in the periodic table, the basis of phosphors and screens.
Reactivity
4 Tb + 3 O₂ → 2 Tb₂O₃
In air it becomes covered with a brown oxide that does not protect the metal. Heated in oxygen it gives the mixed oxide Tb₄O₇, with a mixture of +3 and +4.
2 Tb + 6 H₂O → 2 Tb(OH)₃ + 3 H₂
It reacts with water releasing hydrogen, more vigorously when hot. The finished phosphors, by contrast, are insoluble and stable.
2 Tb + 6 HCl → 2 TbCl₃ + 3 H₂
It dissolves in acids giving colorless salts that glow green under ultraviolet light. The chloride is the route to obtaining the metal.
Tb₂O₃ + 6 HF → 2 TbF₃ + 3 H₂O
With hydrofluoric acid it gives the fluoride, a precursor of optical crystals and of the rare TbF₄ with terbium(IV), a powerful oxidant.
Notable compounds
| Compound | Formula | Use or importance |
|---|---|---|
| Terbium oxide | Tb₂O₃ | Green phosphor for screens and lamps |
| Terbium borate | TbBO₃ | Green for fluorescent lamps and LEDs |
| Terfenol-D alloy | TbDyFe₂ | Actuators that stretch with magnets |
| Terbium fluoride | TbF₃ | Optical crystals and lasers |
| Terbium chloride | TbCl₃ | Production of the metal and phosphors |
Electron shell distribution
View in the atom builderIsotopes
All natural terbium is terbium-159 (¹⁵⁹Tb), with 94 neutrons: it is monoisotopic and stable. The other isotopes are artificial. Terbium-149 (¹⁴⁹Tb, 4 hours) emits alpha particles and is researched for targeted radiotherapy; terbium-161 (¹⁶¹Tb, 6.9 days) combines beta and gamma for therapy and diagnosis.
Occurrence and production
It is scarce, at about 1.2 ppm, in bastnasite, monazite and xenotime alongside gadolinium and dysprosium. It does not form minerals of its own. It is obtained by acid dissolution and fine separation by solvent extraction, and the metal by electrolysis or reduction with calcium in tantalum crucibles.
History and origin of the name
Carl Gustaf Mosander caught a glimpse of it in 1843 in impure yttria and named it terbium, after Ytterby, the Swedish village that gave its name to four elements. Its pure separation took a century because it was confused with erbium and yttrium. Urbain and James isolated it in the 20th century. Television phosphors rescued it from obscurity.
Applications and relevance of terbium
Uses of the element
As an alloy, Terfenol-D with iron and dysprosium stretches in the magnetic field: sonars, fuel injectors and precision actuators vibrate with electricity. It also dopes magnetostrictive steels and calibrates instruments.
Uses of its compounds
The oxide and the borates produce the green of fluorescent lamps, screens and security banknotes. With zirconia it stabilizes high-temperature fuel cells and with other lanthanides it forms magnetocaloric ceramics.
Health, safety and environment
Biological role
It has no biological function and is absorbed very little. Vitrified phosphors do not release terbium in use. Medical isotopes are handled only in hospitals with shielding and dosimetric control.
Safety
Metallic powder can ignite and requires an inert atmosphere and extraction. Finished devices pose no exposure; the risk is industrial, when machining or when handling corrosive fluorides.
Environment
Its low grade forces much rock to be moved per kilo, with waste and reagent consumption. In return, every gram in efficient lamps saves electricity for years. Phosphor recycling recovers terbium along with europium and yttrium.
Frequently asked questions about Terbium
Why does terbium produce green light?
Terbium compounds emit characteristic green light when they absorb energy from a source such as ultraviolet light, electrons or other kinds of radiation. That emission comes from electronic transitions within the terbium ion, which release the absorbed energy in the form of visible light. This property makes it useful as an activator of green phosphors in screens, lamps and display devices.
Is terbium dangerous?
Terbium is not an element present freely in everyday life, and the compounds used in phosphors and devices are usually built into solid materials. However, the pure metal can be reactive and its industrial handling requires precautions, especially with fine powders. For the general public, the relevant exposure is minimal; the risks depend on the chemical form, the concentration and the handling conditions.
What does terbium have to do with nuclear medicine?
Some terbium isotopes, such as terbium-149, are being researched for targeted alpha-particle radiotherapy. These particles can damage tumor cells in a highly localized way, potentially reducing the effect on nearby healthy tissue. These are experimental or translational research applications, not a domestic use of the element. Nuclear medicine thus harnesses very specific radioactive properties of certain isotopes.
What is the difference between terbium and europium in lighting?
Both are lanthanides used in phosphors, but they emit different colors: terbium stands out for its green light, while europium can produce red or blue emissions depending on the compound. In a screen or a lamp, these materials can be combined with other phosphors to obtain white light or specific colors. The choice depends on the desired wavelength, luminous efficacy and the stability of the material.
What is terbium used for in screens and lamps?
Terbium is used in green phosphors for televisions, monitors, fluorescent lamps and other luminescent devices. Its compounds can be combined with red and blue phosphors to generate white light or reproduce a wider color range. It is also used in materials for lasers and in high-efficiency lighting technologies. Its main function is to convert energy into stable, controllable green light.
What is terbium used for in fuel cells?
Terbium oxide can be used together with zirconium dioxide as a crystalline stabilizer in high-temperature fuel cells. This combination helps keep the structural stability of the ceramic material when it is subjected to demanding thermal cycles. Greater stability favors the durability and performance of the component, a key aspect in clean energy and electrochemical conversion technologies.
Where does terbium come from?
Terbium is obtained from rare-earth minerals, especially bastnasite and monazite, where it appears mixed with other lanthanides. After the extraction and chemical treatment of the ore, it is separated by techniques such as solvent extraction and ion exchange. It can then be obtained as terbium oxide, the usual form for manufacturing phosphors, ceramics and optical materials.
Main sources consulted: PubChem, Royal Society of Chemistry, NIST, IUPAC. Last revision: November 2026.