What is Thulium?

Thulium is a chemical element which is a member of the Lanthanide series and is placed in the 6th period. Symbol of the thulium element is Tm. It is a metal which is traditionally considered to be one of the rare earth metals. It's atomic number 69. Mendelevium which is a member of the actinide series is placed below thulium in the periodic table. Another metal erbium is found at the left of thulium and ytterbium is present at the right of it in the sixth period of the periodic table. Erbium and ytterbium metals are also members of the Lanthanide series. Thulium is a member of f – block element. It is the least common rare earth metal. It is found in earth’s crust. For every kg of earth’s crust its occurrence is 500 micrograms. In soil its occurrence is 0.5 parts per million. It exists in concentrations of 1 part per trillion by moles in the solar system. 

Thulium was discovered by Swedish Chemist Per Teodor Cleve in 1879. He discovered it by removing the contaminants from the oxides of rare earth elements. He took erbia (Erbium oxide) and started to remove impurities from it. During his study, he obtained two new substances. One new substance was brown in color, he named it holmia. Holmia is an oxide of the element holmium. Another new substance was green in color and was oxide of another new unknown element. Per Teodor Cleve named this unknown new element thulium and its oxide thulia. 

The word thulium is taken from the Greek word ‘thule’ which is the name of an ancient Greek place. It is associated with Iceland. In ancient Greek and Roman literature, thule is the farthest north location. British chemist Charles James was the first chemist who obtained pure thulium. 

Thulium is not found in pure elemental form in nature as it is a least abundant rare earth metal. Its most common oxidation state is +3 like other lanthanides and rare earth metals. It has a silvery grey appearance and it's not a hard metal. It can be cut by a knife. It is ductile and shows resistance to corrosion. 

Thulium has various isotopes ranging from thulium -145 to thulium – 179. Its most stable isotope which is found abundantly is thulium – 169. It is predicted to undergo – decay and forms 165Ho with a very long half life period. Its synthetic isotope thulium – 171 is also very stable with a half life of 1.92 years. Other synthetic isotopes such as thulium – 167, thulium – 168, thulium – 170 have half - life periods of 9.25 days, 93.1 days and 128.6 days respectively.  

Thulium Atomic Number and Electronic Configuration 

It is the least abundant rare earth metal. Atomic number of thulium is 69. Its electronic configuration is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 4f13 5d0 6s2 or it can be written as [Xe] 4f13 6s2. It has 2 electrons in K – shell, 8 electrons in L – shell, 18 electrons in M – shell and 31 electrons in its outermost shell N, 8 electrons in O shell and 2 electrons in P shell. 

Properties of Thulium

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Physical and Chemical Properties – Physical and chemical properties of thulium are listed below –

  • Pure thulium is a silvery grey lustrous metal. 

  • It tarnishes on exposure to air although it is resistant to corrosion and tarnishes very slowly.

  • It is a soft metal.

  • It is malleable and ductile. 

  • It shows ferromagnetic, antiferromagnetic and paramagnetic properties at specific temperatures. At 32 K, it shows ferromagnetic character while at 32 – 56 K, it shows antiferromagnetic properties. At above 56 K temperature, it is paramagnetic. 

  • It has two major allotropes - alpha thulium and beta thulium. Alpha thulium has tetragonal shape while beta thulium has hexagonal shape. 

  • It has 69 protons in its nucleus. 

  • It is found in solid phase at room temperature. 

  • Its melting point is 1545 .

  • Its boiling point is 1950 .

  • According to the Pauling scale, its electronegativity is 1.25. 

  •  Its oxides are basic in nature. 

  • It shows hexagonal close packed crystal structure. 

  • Its first ionization energy is 596.7 kJ.mol-1.

  • On reaction with chalcogens, it forms thulium chalcogenides. 

  • Reaction with oxygen – Thulium burns easily at 150 temperature and forms its oxide Tm2O3. Reaction is given below –

4Tm + 3O2 → 2Tm2O3

  • Reaction with water – It undergoes hydrolysis with cold water and hot water both. Although it reacts slowly with cold water but very fast with hot water. It forms thulium hydroxide on reaction with water. Reaction is given below –

2Tm(s) + 6H2O(l) → 2Tm(OH)3(aq) + 3H2(g)

  • Reaction with halides – It reacts with halogens and forms thulium halides. It forms different colored halides with different halogens. For example, thulium fluoride is white in color while thulium iodide is yellow in color. In these reactions of thulium with halogens temperature plays a key role. At room temperature thulium reacts with halogens slowly while at higher temperature (> 200 ) it reacts with halogens vigorously. Reactions are given below –

2Tm(s) + 3F2(g) → 2TmF3 (s) 


2Tm(s) + 3Cl2(g) → 2 TmCl3 (s) 


2Tm(s) + 3Br2(g) → 2 TmBr3 (s)


2Tm(s) + 3I2(g) → 2 TmI3 (s) 


  • It dissolves in dilute sulfuric acid and forms a pale green solution containing Tm+3 ions. 

2Tm(s) + 3H2SO4(aq) → 2Tm3+ (aq) + 3SO2−4 (aq) + 3H2(g)

  • Thulium dichloride (thulium halide) reacts vigorously with water. Reaction is given below –

TmCl2 + H2O 🡪 Tm(OH)3 + H2

  • Reaction with HCl – It reacts with HCl and forms hydrogen gas and thulium chloride. 

Tm + 2HCl 🡪 TmCl2 + H2

Applications of Thulium 

Thulium is an expensive and least abundant rare earth metal. So, it has a few applications. Its uses are listed below –

  • It is used in lasers as an active laser medium material with holmium, chromium and yttrium aluminium garnet. It can lase at 2080 nm and is used in military applications, medicines, meteorology etc. 

  • It is used as an X – ray source. These types of X-ray devices are used in medical and dental diagnosis. 170Tm is being used in X – ray devices for cancer treatment. 

  • 170Tm is used for industrial radiography. 

  • It is used in high temperature superconductors. 

  • It can be used in ceramic magnetic materials which are used in microwaves. 

  • It can be used in electricity generating windows which work on the principle of a luminescent solar concentrator. 

Effects on Health 

Thulium is very less toxic but at higher concentrations it can be dangerous. Although insoluble thulium salts are completely nontoxic. It can cause damage to the liver and spleen. Inhalation or ingestion of thulium dust is very harmful. It can cause explosions. 

Thulium: Summary in Tabular Form 




Discovered By 

Swedish Chemist Per Teodor Cleve 

First isolated by 

British chemist Charles James

Atomic number 


Standard atomic weight 


Crystal Structure 

hexagonal close packed (hcp)

State at 20


Melting point 


Boiling point 








Electronic configuration 

1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 4f13 5d0 6s2 or [Xe] 4f13 6s2.

Main properties 

Soft metal.

Can be cut by knife.

Reacts with halogens, water and oxygen.

Magnetic property 

Below 32K -Ferromagnetic 

32K to 56K – antiferromagnetic 

Above 56K – Paramagnetic 

Main use 

In portable X – ray machines. 

In solid state lasers.


Thulium salts are slightly toxic. 

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