Thulium (Tm): Properties, Reactions, and Uses
Introduction
Thulium (Tm) is a rare earth element, specifically a lanthanide, known for its unique optical properties when doped into other materials. Its primary real-world significance stems from its applications in advanced laser technology, especially in medical and industrial fields, and in specialized radiation sources.
CBSE/JEE Quick Revision Notes
- Atomic Number (Z): 69
- Symbol: Tm
- Atomic Mass: 168.934 amu
- Electronic Configuration (Ground State): [Xe] 4f¹³ 6s²
- Common Oxidation State (Valency): +3
- Group: Not assigned a main group number (f-block element, Lanthanide series)
- Period: 6
- Block: f-block
- Nature: Soft, malleable, ductile silvery-grey metal. Reacts slowly with oxygen and water.
Electron Configuration & Bonding Behavior
Thulium’s ground state electron configuration is [Xe] 4f¹³ 6s². The presence of partially filled 4f orbitals is characteristic of lanthanides.
- Full Electronic Configuration:
1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹³ 5s² 5p⁶ 6s² - Oxidation State: The most stable and common oxidation state for Thulium, like most lanthanides, is +3. This state is achieved by the loss of the two 6s electrons and one 4f electron. Although other oxidation states like +2 exist, they are much less common and generally less stable in aqueous solutions.
- Bonding: Due to the relatively large size and low electronegativity, thulium primarily forms ionic compounds. Covalent character is minimal, mainly seen in complexes with highly polarizable ligands.
- Lanthanide Contraction: Thulium exhibits the effect of lanthanide contraction, which results in a smaller-than-expected ionic radius for elements immediately following it in the periodic table (e.g., hafnium), influencing their chemical properties.
Crucial Chemical Reactions
Thulium is a moderately reactive metal. Its reactions typically lead to the formation of Tm(III) compounds.
Reaction with Air/Oxygen
Thulium tarnishes slowly in air and burns readily at 150°C to form thulium(III) oxide.
4 Tm(s) + 3 O₂(g) → 2 Tm₂O₃(s)
Reaction with Water
Thulium reacts slowly with cold water and more rapidly with hot water to form thulium(III) hydroxide and hydrogen gas.
2 Tm(s) + 6 H₂O(l) → 2 Tm(OH)₃(aq) + 3 H₂(g)
Reaction with Halogens
Thulium reacts vigorously with all halogens (fluorine, chlorine, bromine, iodine) to form the corresponding thulium(III) halides.
2 Tm(s) + 3 F₂(g) → 2 TmF₃(s) (Thulium(III) fluoride)
2 Tm(s) + 3 Cl₂(g) → 2 TmCl₃(s) (Thulium(III) chloride)
2 Tm(s) + 3 Br₂(g) → 2 TmBr₃(s) (Thulium(III) bromide)
2 Tm(s) + 3 I₂(g) → 2 TmI₃(s) (Thulium(III) iodide)
Reaction with Acids
Thulium dissolves readily in dilute mineral acids to form Tm(III) salts and hydrogen gas.
2 Tm(s) + 3 H₂SO₄(aq) → Tm₂(SO₄)₃(aq) + 3 H₂(g) (Thulium(III) sulfate)
2 Tm(s) + 6 HCl(aq) → 2 TmCl₃(aq) + 3 H₂(g) (Thulium(III) chloride)
Industrial and Biological Importance
Industrial Importance
- Lasers: Thulium-doped YAG (Tm:YAG) lasers are a significant application. These lasers emit light in the 2-micrometer range, which is strongly absorbed by water. This property makes them highly useful in medical procedures, particularly in urology (e.g., lithotripsy for kidney stones, prostatectomy), dermatology, and ophthalmology, due to precise tissue ablation with minimal collateral damage.
- Portable X-ray Devices: The radioisotope Thulium-170 (¹⁷⁰Tm) is a compact and relatively low-energy gamma radiation source. It is used in portable X-ray devices for non-destructive testing in industrial settings and for medical diagnostic applications in remote areas where conventional X-ray machines are impractical.
- Ceramics: Thulium is used in specialized ceramics and glasses, including those for high-temperature applications or as magnetic materials.
- Magnetic Materials: Minor constituent in certain specialized magnetic alloys.
Biological Importance
- No Known Biological Role: Thulium has no known essential biological role in humans or other organisms.
- Toxicity: Like most heavy metals, thulium compounds should be handled with care. While its toxicity is generally considered low, ingestion or inhalation of soluble thulium compounds can lead to health concerns.