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)
white
2Tm(s) + 3Cl2(g) → 2 TmCl3 (s)
yellow
2Tm(s) + 3Br2(g) → 2 TmBr3 (s)
white
2Tm(s) + 3I2(g) → 2 TmI3 (s)
Yellow
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
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FAQs on Thulium
1. What is Thulium and where is it placed in the periodic table?
Thulium is a chemical element with the symbol Tm and atomic number 69. It is a bright, silvery-grey metal that is soft enough to be cut with a knife. As a member of the lanthanide series, it is located in the f-block of the periodic table, specifically in period 6. It is the second-to-last element in the lanthanide series, positioned between Erbium and Ytterbium.
2. What are the key physical and chemical properties of Thulium?
Thulium exhibits several distinct properties characteristic of a late lanthanide element.
- Appearance: It is a soft, malleable, and ductile metal with a bright silvery lustre.
- Atomic Mass: Its atomic mass is approximately 168.93 u.
- Reactivity: Thulium is relatively stable in air but slowly tarnishes. It reacts slowly with cold water and more quickly with hot water to form thulium hydroxide.
- Oxidation State: Its most common and stable oxidation state is +3, although the +2 state is also known.
3. What is the electronic configuration of Thulium and its most stable oxidation state?
The ground-state electronic configuration of Thulium (Tm), atomic number 69, is [Xe] 4f¹³ 6s². When it forms ions, it typically loses the two 6s electrons and one 4f electron to achieve its most stable oxidation state of +3. The resulting Tm³⁺ ion has the electronic configuration [Xe] 4f¹², which is a stable arrangement for this element.
4. How is Thulium extracted from its natural sources?
Thulium is not found free in nature. It is commercially extracted from minerals like monazite and bastnäsite, which contain small amounts of all rare-earth elements. The extraction process is complex and involves techniques like ion exchange and solvent extraction to separate Thulium from other more abundant lanthanides. The pure metal is then typically obtained by the reduction of its anhydrous fluoride (TmF₃) with calcium metal.
5. What are the main uses of Thulium in technology and medicine?
Despite its rarity, Thulium has several highly specialised applications:
- Portable X-ray Devices: When bombarded with neutrons in a nuclear reactor, Thulium forms the isotope Tm-170. This isotope emits X-rays and is used as a radiation source in lightweight, portable X-ray machines.
- Lasers: Thulium is used to dope yttrium aluminium garnet (YAG) crystals to create solid-state lasers (Tm:YAG lasers), which are effective for surgical procedures.
- Ceramic Magnets: It is used in the production of ferrites, which are ceramic magnetic materials found in microwave equipment.
6. Why is Thulium considered so rare and valuable?
Thulium is the least abundant of all the stable lanthanide elements in the Earth's crust. Its scarcity is the primary reason for its high cost and value. Its low concentration in ores like monazite (around 0.007%) makes the separation and purification process extremely difficult and expensive compared to other rare-earth metals. This combination of natural scarcity and a complex extraction process makes Thulium one of the most valuable rare-earth elements.
7. Is Thulium considered toxic or harmful to humans?
Soluble thulium salts are considered to be mildly toxic when ingested, but insoluble thulium compounds are regarded as non-toxic. The pure metal itself poses a low toxicity risk. In general, Thulium has no known biological role in the human body, and due to its rarity and specialised uses, most people will not encounter it in a way that would pose a health risk. Standard laboratory precautions should be taken when handling its compounds.
8. How does Thulium's position as a lanthanide affect its properties due to lanthanoid contraction?
As one of the last elements in the lanthanide series, Thulium is significantly affected by lanthanoid contraction. This phenomenon is the steady decrease in the size of atoms and ions of the lanthanides with increasing atomic number. For Thulium, this results in a higher density, hardness, and melting point than would otherwise be expected. The poor shielding of the nuclear charge by the 4f electrons causes the outer 6s electrons to be drawn closer to the nucleus, leading to a smaller atomic radius and these distinct physical properties.
