Atomic number 69

Thulium

Tm
+ phase
− phase

Shells Orbitals 4f 6s

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69 electrons in 6 shells: 2, 8, 18, 31, 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: 168.93(1) Da
Electron configuration: [Xe] 4f¹³ 6s²
Phase at 25 °C: solid
Melting point: 1545 °C
Boiling point: 1950 °C
Density: 9.32 g/cm³
Electronegativity (Pauling): 1.25
Discovered: 1879, Per Theodor Cleve

Neighbours in the table

Er Yb

Thule, the earliest name for Scandinavia in Los Alamos’s words, gave thulium its name. CIAAW, the atomic-weights commission of IUPAC, describes Thule as the northernmost part of the civilised world. The Swedish chemist Per Theodor Cleve found the element in 1879 in a sample of an erbium mineral. He worked the way Carl Gustaf Mosander had worked on lanthanum, erbium and terbium, hunting for impurities hidden in the oxides of other rare earths, in Jefferson Lab’s account.

Cleve began with erbia, the oxide Er₂O₃, and removed every contaminant then known. What remained split into two new materials, one brown and one green. He called the brown oxide holmia and the green one thulia, and thulia proved to be the oxide of thulium. Thulium itself was first isolated 32 years later, in 1911, by the American chemist Charles James, CIAAW records.

A thulium atom holds thirteen 4f electrons and two 6s electrons beyond its xenon core, [Xe] 4f¹³ 6s². Jefferson Lab lists +3 as its oxidation state, a charge assigned by treating each bond as ionic, and Los Alamos gives +2 and +3. Three of its compounds, the oxide Tm₂O₃, the fluoride TmF₃ and the iodide TmI₃, carry thulium at +3.

Where it is found

Among the naturally occurring rare-earth elements, thulium is the least abundant, Jefferson Lab reports, at an estimated 0.52 milligrams per kilogram of the Earth’s crust. Seawater carries about 1.7 × 10⁻⁷ milligrams per litre. Even so, with new sources recently found, Los Alamos now ranks thulium roughly with silver, gold or cadmium for rarity.

The element occurs in small amounts alongside other rare earths in several minerals. Its commercial source is monazite, a phosphate written (Ce, La, Th, Nd, Y)PO₄, which holds about 0.007 per cent thulium in Los Alamos’s figure. Ion exchange now draws thulium out of monazite sand, Jefferson Lab reports. The metal is won by reducing the oxide with lanthanum metal, or with calcium in a closed container, Los Alamos reports.

Thulium comes out of rare-earth mining as a whole, an estimated 390,000 tons of oxide equivalent worldwide in 2024, in US Geological Survey figures.

Why it matters

Thulium metal is relatively costly and became available only recently, Jefferson Lab notes, and Los Alamos puts its few practical applications down to that high price. Natural thulium is entirely stable thulium-169. In a nuclear reactor a thulium-169 nucleus captures a neutron and becomes thulium-170, which emits X-rays, in IUPAC’s isotope notes.

Thulium-170 has been used as the radiation source in portable X-ray equipment, in IUPAC’s notes. The petrochemical industry uses it in industrial radiography of welded pipes and tanks, IUPAC adds. Its half-life is 128.6 days in the evaluation of the International Atomic Energy Agency (IAEA). Of its decays, 99.869 per cent are beta-minus decays and 0.131 per cent electron capture, in IAEA’s evaluation.

Thulium-170 has served in brachytherapy, treatment from a radioactive source placed inside the body, and in radiosynovectomy, radiation treatment of medium-sized joints, IUPAC records. Thulium-167, whose half-life is 9.25 days in the Atomic Mass Data Center list on PubChem, is useful for tumour and bone studies. Thulium-171 may prove useful as an energy source, in Los Alamos’s view.

Natural thulium may find use in ferrites, ceramic magnetic materials in microwave equipment, and it can dope fibre lasers, Los Alamos reports. Thulium itself forms no commercially important compounds, Jefferson Lab adds.

Properties

Properties of thulium 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	69
Standard atomic weight	168.93(1) Da	Da	IUPAC/CIAAW 2024
Melting point	1545 °C	°C	Los Alamos: 1545 °C
Boiling point	1950 °C	°C	Los Alamos: 1950 °C
Density	9.32 g/cm³	g/cm³
Electronegativity (Pauling)	1.25
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹³
Phase at 25 °C	solid

Safety

Thulium metal is classed as a flammable solid in 94% of company reports to the European Chemicals Agency, under the signal word Danger. In 80% of reports it also causes serious eye irritation and may cause respiratory irritation, while 6% find that it meets no hazard criteria. Haz-Map and Los Alamos rate its acute toxicity low to moderate. Natural thulium is stable, so any radiation hazard belongs to reactor-made isotopes such as thulium-170, and the PubChem Laboratory Chemical Safety Summary for thulium gives the handling guidance.

Questions

Is thulium radioactive?

Natural thulium is not radioactive. Every atom of it is thulium-169, a stable isotope, Los Alamos states, and it is the element's only stable isotope, Jefferson Lab adds. Los Alamos counts 25 known isotopes with atomic masses from 152 to 176, while the IAEA Atomic Mass Data Center list on PubChem runs from thulium-144 to thulium-182. The radioactive isotope thulium-170 is made by bombarding thulium-169 with neutrons in a nuclear reactor, in IUPAC's isotope notes.

What is thulium used for?

Not much, because the metal costs a lot, Los Alamos explains. Its most practical role comes after a stay in a nuclear reactor: the thulium-170 made there gives off X-rays and has been used as the source in portable X-ray units and in checks of welded pipes and tanks for the petrochemical industry. Thulium may also find use in microwave ferrites and can dope fibre lasers, Los Alamos reports.

Is thulium the rarest rare earth element?

Among the rare-earth elements that occur naturally, thulium is the least abundant, according to Jefferson Lab, which puts it at an estimated 0.52 milligrams per kilogram of crust. Scarcity is relative, though. Since new deposits came to light, Los Alamos places thulium's rarity near that of silver, gold or cadmium, and it is extracted from monazite, which holds roughly 0.007 per cent of it.

Where does the name thulium come from?

Thule was the earliest name for Scandinavia, which CIAAW, IUPAC's atomic-weights commission, calls the northernmost part of the civilised world. Per Theodor Cleve, the Swedish chemist who found the element in 1879, gave the name first to a green oxide, thulia. The element itself waited until 1911, when the American chemist Charles James first isolated it, in CIAAW's history.