Thulium (Tm)


Thulium is a chemical element with atomic number 69 in the periodic table. It is found in Earth’s crust in almost the same quantities as the elements iodine and antimony, with an occurrence of around 500 parts per million. 

Being a member of the lanthanide series of periodic table elements, this rare earth metal has three valence electrons. Thulium is present in the human body as a trace amount in the kidneys, the liver, and the bones. Despite having limited practical use, thulium isotopes are most beneficial in nuclear medicine.

Fact Box

Chemical and Physical Properties of Thulium

The symbol in the periodic table of elements: Tm

Atomic number: 69

Atomic weight (mass): 168.934 g.mol-1

Group number: Lanthanides

Period: 6 (f-block)

Color: A bright, silvery metal at room temperature

Physical state: Solid at 20°C

Half-life: From 3.1(3) milliseconds to 1.92 years

Electronegativity according to Pauling: 1.2

Density: 9.3 at 20°C

Melting point: 1545°C, 2813°F, 1818 K

Boiling point: 1950°C, 3542°F, 2223 K

Van der Waals radius: Unknown

Ionic radius: 1.59 (+1) Å

Isotopes: 34

Most characteristic isotope: 169Tm

Electronic shell: [Xe] 4f136s2

The energy of the first ionization: 595.3 kJ.mol-1

The energy of the second ionization: 1160.7kJ.mol-1

The energy of the third ionization: 2284 kJ.mol-1

Discovery date: In 1879 by Per Teodor Cleve

With the periodic table symbol Tm, atomic number 69, relative atomic mass of 168.934  g.mol-1, and electron configuration X, thulium is a moderately soft and ductile metal with bright, silvery color and a close-packed hexagonal structure. It reaches its boiling point at 1950°C, 3542°F, 2223 K, while the melting point is achieved at 1545°C, 2813°F, 1818 K. This member of the lanthanide family of elements in the periodic table has an electronegativity of 1.2 according to Pauling, whereas the atomic radius according to van der Waals is unknown. 

When exposed to temperatures above 56 K (−217°C, or −359°F), thulium becomes a strong paramagnet. On the other hand, exposure to temperatures between 56 and 32 K (−241°C, or −402°F) give thulium metal antiferromagnetic properties, while below 32 K (-241.15°C, or -402.07°F) thulium adopts the characteristics of a ferrimagnet.     


How Was Thulium Discovered?

Being familiar with the fact that many of the rare earth elements were obtained from a yttrium sample, Per Teodor Cleve was conducting his research experiments on an erbia (erbium oxide) sample in his laboratory at the University of Uppsala, Sweden. Cleve believed that the elements isolated from this mineral are further contaminated with traces of new rare earth elements. And – he was right.

The Discovery of Per Teodor Cleve

After the elements erbium and terbium were extracted from yttrium in 1843, this Swedish chemist explored the erbium oxide more closely to confirm his scientific conviction. His dedicated work resulted in the discovery of element 69, i.e. thulium (Tu) in 1879.

In fact, it wasn’t just thulium that he discovered. His experiments produced two new substances – a green and a brown one. Cleve labeled the green substance as ‘thulia’ (thulium oxide), while the brown one turned out to be ‘holmia’, i.e. an oxide of holmium (element 67).

The Contribution of Charles James

In 1911, the British-American researcher Charles James succeeded in isolating the pure, elemental form of thulium by performing 15.000 recrystallizations of the thulium bromate compound. 

How Did Thulium Get Its Name?

Element 69 was named ‘thulium’ after Thule, the ancient name for Scandinavia. By proposing the name of a mythological region also known as Ultima Thule, the discoverer of thulium honored his native country. 

Where Can You Find Thulium?

All rare-earth elements occur alongside other rare-earth elements (lanthanides) in nature, and so does thulium. Small quantities of this chemical element can be traced in the minerals monazite (0.007% of thulium occurrence) and bastnasite (0.0008% of thulium occurrence). Thulium also occurs in the minerals gadolinite, xenotime, and euxenite. 

The largest mining locations of thulium ores in the world can be found in China, Australia, United States, Brazil, India, and Sri Lanka. 

Thulium in Everyday Life

Element 69 is largely used in medicine, but it can be also applied in other industrial branches:

  • Thulium is one of the main chemicals used in the manufacturing of medical lasers with surgical applications;
  • Element 69 is also applied as radiation source as a dopant in yttrium-aluminum garnet for laser applications;
  • After irradiation in a nuclear reactor, thulium isotopes become a rich energy source of x-rays. As a radiation source, they are further used in the production of lightweight, portable x-ray machines used in radiology for early detection of changes within the human body;
  • Element 69 has potential use in the manufacturing of ceramic magnetic materials, used in the production of microwave equipment;
  • This chemical element is applied in high-temperature superconductors in a similar way yttrium is used.

How Dangerous Is Thulium?

Thulium is generally considered as a non-toxic substance. However, while the soluble salts of thulium are not hazardous, the insoluble salts of element 69 can be extremely toxic. The powdered form of thulium may pose a fire or explosion hazard. 

Environmental Effects of Thulium

This chemical element has no known biological role. For this reason, it does not pose any danger to the geological, biological, or aquatic systems of our environment. 

Isotopes of Thulium

There are 34 isotopes of thulium observed. The natural thulium is made up of a single stable isotope (169Tm). Also, there are at least 16 radioactive isotopes of this chemical element with half-lives less than two minutes. 

Thulium-170 isotope is the most exploited form of element 69 in nuclear medicine. It’s synthesized by bombarding thulium in a nuclear reactor;

Isotopes before the most stable thulium-169 form decay into erbium isotopes via electron capture as a primary decay mode, while the primary mode after is beta emission by which thulium isotopes decay into ytterbium isotopes.

Main isotopes of thulium

Iso­tope Decay
abun­dance half-life (t1/2) mode pro­duct
167Tm syn 9.25 d ε 167Er
168Tm syn 93.1 d ε 168Er
169Tm 100% stable
170Tm syn 128.6 d β 170Yb
171Tm syn 1.92 y

Source: Wikipedia

List of Thulium Compounds 

In compounds, thulium is typically prepared in the +3 oxidation states. Such is the dark-colored compound diiodide TmI2. When this chemical adopts the stable +3 state, it forms a series of pale green salts.

Thulium is a reactive metal. It slowly tarnishes when exposed to both air and H2O. In a reaction with oxygen, it forms thulium (III) oxide (Tm2O3):

 4Tm + 3O2 → 2 Tm2O3

Thulia, or thulium (III) oxide, forms a strong chemical bond with hydrogen chloride. The result of this chemical reaction is the production of hydrogen gas and thulium chloride.

When exposed to temperatures above 200°C, this chemical forms TmF3 (white), TmCl3 (yellow), TmBr3 (white), and TmI3 (yellow) in a reaction with halogen elements. 

The list of most often prepared thulium compounds includes the following items:

  • Thulium(II) chloride
  • Thulium(III) bromide
  • Thulium(III) chloride
  • Thulium(III) fluoride
  • Thulium(III) hydroxide
  • Thulium(III) oxide

5 Interesting Facts and Explanations

  1. American chemist Theodore William Richards is the first American scientist to receive the Nobel Prize in Chemistry for accurately determining the atomic weights of many chemical elements, including the one of thulium. 
  2. After the element promethium, thulium is the second rarest lanthanide that occurs in Earth’s crust.
  3. For commercial purposes, thulium was prepared for the first time in the late 1950s, by using ion-exchange separation technology. The thulium metal can be isolated by the method of reduction of its oxide with lanthanum metal, or by calcium reduction in a closed container.
  4. Holmium-chromium-thulium triple-doped yttrium aluminum garnet (Ho:Cr:Tm:YAG, or Ho,Cr,Tm:YAG) is an active laser medium material with high efficiency.
  5. Tm3+ ions radiate a bright blue luminescence when exposed to ultraviolet light. Hence, thulium is applied in Euro banknotes to protect against counterfeiting.