← Periodic table · Lanthanides
Thulium (Tm)
Thulium (symbol Tm) is the chemical element with atomic number 69: a lanthanide (rare earth), solid at 0 °C, with an atomic mass of 168.93 u. Discovered in 1879 (Per Teodor Cleve).
Thulium is one of the least abundant lanthanides, which makes it costly and little used. It turns up in surgical lasers and portable X-ray machines. Its name evokes Thule, the mysterious far-northern land of the ancients.
Key facts
- Atomic number
- 69
- Symbol
- Tm
- French name
- Thulium
- Atomic mass
- 168.93 u
- Molar mass
- 168.93 g/mol
- Family
- Lanthanides
- Position
- period 6, f-block
- State at 0 °C
- Solid
- Melting point
- 1,544.9 °C
- Boiling point
- 1,949.9 °C
- Density
- 9.32 g/cm³
- Electronegativity (Pauling)
- 1.25
- First ionization energy
- 597 kJ/mol
- Atomic radius
- 175 pm
- Electron configuration
- [Xe] 4f¹³ 6s²
- Abundance in Earth's crust
- 0.52 ppm (62nd most abundant element)
- Indicative price
- 3,000 $/kg (order of magnitude)
- Radioactive
- no
- Discovery
- 1879, Per Teodor Cleve
Origin of the name
From Thule, the ancient name for a land in the far north.
What does thulium look like?
A silvery-gray metal soft enough to cut with a knife, fairly stable in air.
Where is thulium found?
About 0.5 ppm of Earth's crust: rare for a rare earth, yet still more abundant than silver. It accompanies the other heavy rare earths in xenotime and ionic clays.
Natural sources
- Xenotime
- Euxenite
- Monazite
- Ion-adsorption clays
How is thulium produced?
Separated by ion exchange, then reduced with lanthanum. World production is limited to a few tonnes a year.
Top producers
- China
- Myanmar
What is thulium used for?
- Thulium-doped fiber lasers (2 micrometers) in urological surgery.
- Thulium-170, an X-ray source for portable radiography devices.
- Research into magnetic and superconducting materials.
In everyday life
- Laser treatment of the prostate or urinary stones
Role in living things
No known biological role.
Hazards of thulium
Low toxicity; flammable dust. Thulium-170 is radioactive.
Cosmic origin
- Neutron stars: Created when two neutron stars collide — the “forge” of gold.
History of thulium
Per Teodor Cleve identified it in 1879 in erbia and named it after Thule, a legendary land of the far north. In 1911 the American Charles James obtained pure thulium after some 15,000 successive recrystallizations of thulium bromate.
Did you know?
- Charles James's 15,000 recrystallizations show just how hard rare earths are to separate.
- Thulium-170 emits X-rays with no power supply, handy for radiography in the field.
- It is the second least abundant lanthanide, after radioactive promethium.
Isotopes of thulium
Thulium has only one stable isotope, thulium-169. Irradiated in a reactor, it becomes thulium-170, a portable source of X-rays.
- thulium-167: radioactive, half-life 9.25 days
- thulium-169: 100% of natural thulium, stable
- thulium-170: radioactive, half-life 129 days. A compact X-ray source for portable radiography, no electricity needed.
- thulium-171: radioactive, half-life 1.92 years
Related elements
Same family (lanthanides): Lanthanum (La), Cerium (Ce), Praseodymium (Pr), Neodymium (Nd), Promethium (Pm), Samarium (Sm), Europium (Eu), Gadolinium (Gd), Terbium (Tb), Dysprosium (Dy), Holmium (Ho), Erbium (Er), Ytterbium (Yb), Lutetium (Lu).