Thulium

Thulium is the second rarest naturally occurring lanthanide and one of the least commonly encountered. It produces radiation useful in portable medical X-ray machines and has some specialised laser applications. Its name comes from Thule, the ancient Greek name for the northern extremes of the world.

• Atomic Number6969 protons, 69 electrons
• Atomic Mass168.93422 u69× heavier than hydrogen
• State at Room TempSolidSolid
• Density9.32 g/cm³
• Melting / Boiling1544.8°C / 1949.8°C
• Discovered1879

What is Thulium?

Thulium is a lanthanide rare earth metal with 69 protons. Thulium-170 emits X-rays and was used in portable X-ray units for field hospitals before small electronic X-ray sources became practical. Thulium also has applications in specialised fibre lasers emitting around 2000 nm. It was isolated by Per Teodor Cleve in 1879.

Fact Thulium is element 69 in the periodic table, symbol Tm. As a lanthanide, it is part of the group of 15 elements sharing very similar chemistry, formed by the filling of the 4f electron subshell. All lanthanides form stable +3 ions and are found together in rare earth mineral deposits.

Where you find Thulium

On Earth

Thulium is found alongside other rare earth elements in minerals such as monazite, bastnäsite and xenotime. China produces the vast majority of world supply, with smaller contributions from Australia, the United States, Russia and India. It is never found as a free metal in nature.

• Monazite and bastnäsite. The primary rare earth minerals that contain Thulium.
• Ion-adsorption clays. Certain clay deposits in southern China are particularly rich in heavier lanthanides including Thulium.

How we use Thulium

Thulium-170 emits X-rays and was used in portable X-ray units for field hospitals before small electronic X-ray sources became practical. Thulium also has applications in specialised fibre lasers emitting around 2000 nm. It was isolated by Per Teodor Cleve in 1879.

Did you know? Thulium was one of the last lanthanides to be isolated in pure form. The extraordinary chemical similarity between adjacent lanthanides made separation extraordinarily difficult until ion exchange chromatography was developed in the 1940s.

How it was discovered

Thulium was identified and separated from the mixture of rare earth elements found in minerals from Ytterby, Sweden and other locations, through painstaking fractional crystallisation and spectroscopic analysis over many decades in the 19th century.

Deeper dive: thulium and rare earth supply chains

The lanthanides, often called rare earth elements, are critically important for clean energy technologies. Neodymium and praseodymium go into the powerful magnets in EV motors and wind turbines. Dysprosium improves those magnets at high temperatures. Lanthanum and cerium go into NiMH batteries, catalysts and glass. Europium and terbium provide red and green in LED phosphors. This means that the global transition to clean energy depends heavily on rare earth elements, and their supply is dominated by China, which produces over 60% of the world's rare earth output. Concerns about supply security have spurred investment in rare earth mining projects in Australia, Canada, the USA and elsewhere.

Moving to 70 protons brings us to the next element on the periodic table.