Thulium
Atomic Data
| Atomic Number | 69 |
| Symbol | Tm |
| Atomic Weight | 168.93 u |
| Density (STP) | 9.321 g/cm³ |
| Melting Point | 1544.85 °C (1818 K) |
| Boiling Point | 1949.85 °C (2223 K) |
| Electronegativity | 1.25 (Pauling) |
| Electron Config. | 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f13 5s2 5p6 6s2 |
| Oxidation States | +2, +3 |
| Phase at STP | Solid |
| Category | Lanthanoid |
| Period / Group | 6 / None |
| CAS Number | 7440-30-4 |
Electron Configuration
[Xe] 4f13 6s2
| Shell | n | Subshell | Electrons | Cumulative |
|---|---|---|---|---|
| K | 1 | 1s | 2 | 2 |
| L | 2 | 2s | 2 | 4 |
| L | 2 | 2p | 6 | 10 |
| M | 3 | 3s | 2 | 12 |
| M | 3 | 3p | 6 | 18 |
| M | 3 | 3d | 10 | 28 |
| N | 4 | 4s | 2 | 30 |
| N | 4 | 4p | 6 | 36 |
| N | 4 | 4d | 10 | 46 |
| N | 4 | 4f | 13 | 59 |
| O | 5 | 5s | 2 | 61 |
| O | 5 | 5p | 6 | 67 |
| P | 6 | 6s | 2 | 69 |
| Total | 69 | 69 | ||
Isotopes of Thulium
Thulium is monoisotopic: ¹⁶⁹Tm is its only naturally occurring stable isotope, accounting for 100% of all natural Thulium.
| Isotope | Symbol | Protons | Neutrons | Abundance | Stability |
|---|---|---|---|---|---|
| Thulium-169 | ¹⁶⁹Tm | 69 | 100 | 100 | Stable |
Abundance & Occurrence
Thulium is present in Earth's crust at approximately 0.52 ppm by mass and at approximately 0.01 ppm by mass throughout the universe.
Earth's Crust (ppm by mass)
Universe (ppm by mass)
Discovery & History
Read more about the discovery of the periodic table of elements →
Safety & Handling
- Dust inhalation: Thulium metal dust and oxide are respiratory irritants; fine particulate from processing should not be inhaled: use ventilation and respiratory protection.
- Thulium-170: radiation: Tm-170 (t½ = 129 days, beta/gamma) is used as a portable X-ray source; it must be handled in shielded containers and requires radiation work licences, dosimetry, and regulated disposal.
- Tm:YAG and Tm:fibre laser hazards: Thulium lasers (~2 µm) cause serious eye and tissue injuries without appropriate beam controls; Class 4 laser safety procedures are mandatory.
- Fire hazard: Thulium metal powder is flammable; metal fires require Class D extinguishing agents.
Thulium in the Real World
Real-World Uses
- Portable X-ray sources: Thulium-170 (t½ = 128.6 d, 84 keV X-ray) compact radioactive sources are used in handheld field radiography units to inspect welds and castings in remote locations where AC power for X-ray tubes is unavailable.
- Thulium fibre lasers: Tm³-doped silica fibre lasers emit at ~1.9–2.05 μm and deliver multi-watt outputs used in material processing, eye-safe atmospheric LIDAR, and medical applications including laser coagulation in urology and ENT surgery.
- Doped crystal and glass research: Thulium-doped materials (YAG, silica, fluoride fibres) are actively studied for upconversion lasers (infrared to visible photon conversion), photon avalanche effects, and blue-emitting solid-state lasers.
Downloadable Resources
Free periodic table reference sheets for classrooms, study sessions, and laboratory use.
Frequently Asked Questions
What is thulium used for?
Thulium-170, a radioactive isotope produced in nuclear reactors, is used in portable X-ray sources for field medicine and industrial inspection where mains power is unavailable. Thulium-doped yttrium aluminium garnet lasers (Tm:YAG) emit at around 2 micrometres, useful in soft-tissue surgery and atmospheric LIDAR. Thulium oxide is used in high-refractive-index glass.
Is thulium the rarest stable element?
Thulium is the rarest of the stable rare-earth elements and one of the rarest stable elements overall, with an abundance of about 0.45 parts per million in Earth's crust: comparable to iodine or arsenic. It has no large concentrated ore deposits and is produced only in small quantities as a byproduct of other rare-earth processing. Only radioactive elements and man-made elements are less abundant.
How was thulium discovered?
Thulium was discovered in 1879 by Swedish chemist Per Teodor Cleve. Working with erbium oxide, he detected impurities and after careful separation isolated two new elements: holmium and thulium. He named thulium after Thule, the ancient Greek and Roman name for the northernmost lands (identified with Scandinavia), reflecting Cleve's Scandinavian heritage.
How does a thulium portable X-ray source work?
Thulium-170 is produced by neutron irradiation of thulium-169 in a nuclear reactor. Tm-170 decays by emitting beta particles and gamma rays (including an 84 keV X-ray), with a half-life of 128.6 days. A sealed source containing Tm-170 emits X-rays continuously without external power. This makes it useful for field radiography: inspecting welds in remote pipelines or providing X-ray capability to field hospitals where electricity is not available.