Atomic number 65

Terbium

Tb
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Shells Orbitals 4f 6s

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65 electrons in 6 shells: 2, 8, 18, 27, 8, 2.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Lanthanide
Period: 6
Block: f
Standard atomic weight: 158.93(1) Da
Electron configuration: [Xe] 4f⁹ 6s²
Phase at 25 °C: solid
Melting point: 1356 °C
Boiling point: 3123 °C
Density: 8.23 g/cm³
Discovered: 1843, Carl Gustaf Mosander

Neighbours in the table

Gd Dy

The name terbium first belonged to a different substance from the one that carries it now. The Swedish chemist Carl Gustaf Mosander resolved an yttrium salt into three elements in 1843, according to CIAAW, the atomic-weights commission of IUPAC, chemistry’s world body. He kept yttrium for one, called a rose-coloured salt terbium and a deep-yellow peroxide erbium. The name terbium comes from Ytterby, the Swedish village where the mineral ytterbite, terbium’s source, was first found.

The labels soon moved. In 1862 the Swiss chemist Marc Delafontaine looked at yttrium again and found the yellow peroxide, CIAAW records. Because erbium by then named the rose-coloured oxide, he revived terbium as the name for the yellow one. The reversal was complete by 1877, in Jefferson Lab’s account, after chemists confused two substances alike in name and properties.

Beyond its xenon core, a terbium atom holds nine 4f electrons and two 6s electrons, written [Xe] 4f⁹ 6s². The f block sits below the main table for compactness, though in Chem1’s account it belongs between lanthanum and hafnium. The metal is silver-grey, malleable and ductile, and soft enough to cut with a knife, Los Alamos reports. It has two crystal forms, which interconvert at 1289 °C.

Where it is found

Terbium makes up 1.2 milligrams per kilogram of the Earth’s crust, Jefferson Lab estimates, and seawater holds about 1.4 × 10⁻⁷ milligrams per litre. Cerite and gadolinite also carry it, according to Los Alamos. Monazite, at about 0.03 per cent terbium, is the commercial source, along with xenotime and euxenite, a complex oxide with 1 per cent or more of terbia, terbium’s oxide. Most terbium is drawn from monazite sand by ion exchange, a resin-based separation, in Jefferson Lab’s account.

Natural terbium is a single stable isotope, terbium-159. The longest-lived radioactive one, terbium-158, has a half-life, the time for half its atoms to decay, of 180 years in the evaluation of the International Atomic Energy Agency (IAEA). There 83.4 per cent of its decays emit a positron, the electron’s positive counterpart, and 16.6 per cent emit an electron. Known terbium isotopes run from terbium-135 to terbium-174 in the IAEA’s evaluation.

Terbium metal is obtained when calcium reduces the anhydrous fluoride or chloride in a tantalum crucible, Los Alamos reports. Vacuum remelting then removes calcium and tantalum impurities.

Why it matters

Terbium has found few uses, Los Alamos notes. Sodium terbium borate goes into solid-state devices and, in Jefferson Lab’s account, produces laser light. Paired with zirconium dioxide, ZrO₂, terbium serves as a crystal stabiliser in fuel cells that run at high temperature. Terbia (Tb₂O₃) could potentially activate green phosphors, materials that glow when struck by electrons or light, in television tubes, according to Jefferson Lab.

Two radioactive isotopes are being used in cancer therapy, according to IUPAC’s isotope notes on PubChem’s record. Terbium-149, with a half-life of about 4.1 hours, is used in targeted radiotherapy that relies on its alpha particles. Only 16.7 per cent of its decays emit an alpha particle, in the IAEA’s evaluation, while the other 83.3 per cent emit positrons. Terbium-161, with a half-life of about 6.9 days, is attached to a carrier molecule that takes it to cancer cells. Gamma cameras can follow where it spreads in the body.

Terbium oxide of at least 99.99 per cent purity cost 670 dollars a kilogram in 2020, the US Geological Survey reports. By 2022 the price had tripled to 2,051 dollars, before falling to an estimated 810 in 2024. That 2024 price was more than three times the 260 dollars listed for dysprosium oxide and over fourteen times neodymium oxide’s 56.

Properties

Properties of terbium Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	65
Standard atomic weight	158.93(1) Da	Da	IUPAC/CIAAW 2024
Melting point	1356 °C	°C	Los Alamos: 1356 °C
Boiling point	3123 °C	°C	Los Alamos: 3123 °C
Density	8.23 g/cm³	g/cm³
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f⁹
Phase at 25 °C	solid

Safety

Elemental terbium is classed as a flammable solid in 79% of the company reports behind its label under the Globally Harmonized System (GHS), which carries the signal word Danger. The remaining 21%, 13 of 62 reports, find that it meets no hazard criteria. Its toxicity is little known, according to Haz-Map and Los Alamos, and natural terbium is not radioactive. The PubChem Laboratory Chemical Safety Summary for terbium sets out the precautions for handling it.

Questions

Is terbium radioactive?

Natural terbium is not. Every terbium atom found in nature is terbium-159, its single stable isotope, according to PubChem's element record. Of its radioactive isotopes, the longest-lived, terbium-158, has a half-life of 180 years in the evaluation of the International Atomic Energy Agency. Terbium-149 and terbium-161, which last hours and days, are the ones used in cancer therapy.

What is terbium used for?

Terbium serves as a dopant in some kinds of solid-state device, Jefferson Lab reports, and with zirconium dioxide it stabilises the crystals of fuel cells that run hot. Sodium terbium borate, one of its compounds, is used to make laser light. Two radioactive isotopes, terbium-149 and terbium-161, are being used in cancer therapy, and terbium-161 can be tracked in the body with gamma cameras, in IUPAC's isotope notes.

Where does terbium come from?

Terbium is recovered from rare-earth ores. Monazite sand is its main commercial source, according to Jefferson Lab, with xenotime and euxenite also worked. World rare-earth mining reached an estimated 390,000 tons of oxide equivalent in 2024, the US Geological Survey reports, and China's production quota accounted for 270,000 tons of that total.