Introduction to Thulium
Thulium (Tm) is a chemical element with atomic number 69. It is classified as a lanthanide, which is a group of elements often referred to as rare earth metals. Despite this designation, thulium, like other rare earths, is not exceptionally rare in the Earth’s crust when compared to truly scarce elements; however, it is sparsely distributed and difficult to extract in pure form. It is the second least abundant of the lanthanides, with only promethium being less common. Thulium is a soft, malleable, and ductile metal with a bright silvery-grey luster. Its primary sources are rare earth mineral deposits found predominantly in countries such as China, Russia, and India.
Occurrence and Properties
Thulium is typically found in minerals such as monazite, xenotime, and euxenite, often co-occurring with other rare earth elements. Its atomic mass is approximately 168.93 g/mol. As a trivalent lanthanide, thulium typically forms compounds where its oxidation state is +3.
Chemical Reactivity of Thulium
Thulium is a moderately reactive metal, exhibiting similar chemical behaviors to other early lanthanides, though it is slightly less reactive than the alkali and alkaline earth metals. Its reactivity stems from its tendency to readily lose its three valence electrons to form stable Tm³⁺ ions.
Reaction with Air
When exposed to air, thulium metal slowly tarnishes, forming thulium(III) oxide (Tm₂O₃). This process is accelerated in moist air. When heated, thulium burns vigorously in air to form the same oxide. This reaction is characteristic of many reactive metals.
The chemical equation for this reaction is: $4\text{Tm(s)} + 3\text{O}_2\text{(g)} \rightarrow 2\text{Tm}_2\text{O}_3\text{(s)}$
Reaction with Water
Thulium reacts slowly with cold water to form thulium(III) hydroxide (Tm(OH)₃) and hydrogen gas. The reaction is considerably more vigorous with hot water or steam, producing a faster release of hydrogen gas. This behavior is typical for moderately reactive metals.
The chemical equation for the reaction with water is: $2\text{Tm(s)} + 6\text{H}_2\text{O(l)} \rightarrow 2\text{Tm(OH)}_3\text{(aq)} + 3\text{H}_2\text{(g)}$
Reaction with Acids
Thulium reacts readily with dilute acids, such as hydrochloric acid (HCl) or sulfuric acid (H₂SO₄), to produce thulium(III) salts and hydrogen gas. The reaction can be quite vigorous due to the liberation of hydrogen.
For example, with dilute hydrochloric acid: $2\text{Tm(s)} + 6\text{HCl(aq)} \rightarrow 2\text{TmCl}_3\text{(aq)} + 3\text{H}_2\text{(g)}$
Safety Profile: Toxicity, Radioactivity, and Flammability
Understanding the safety characteristics of an element is crucial, particularly for rare materials like thulium.
Toxicity
Thulium metal and its compounds are generally considered to have low acute toxicity. However, like many metal powders, thulium dust can be an irritant to the respiratory system if inhaled and to the skin and eyes upon contact. There is limited data on long-term exposure effects, and therefore, proper handling precautions, such as avoiding ingestion and inhalation, are recommended.
Radioactivity
Natural thulium consists entirely of one stable isotope, Thulium-169 ($^{169}\text{Tm}$), and is therefore not naturally radioactive. However, artificial radioisotopes of thulium exist, such as Thulium-170 ($^{170}\text{Tm}$). This isotope is produced by neutron activation and is a beta-emitter with a half-life of 128.6 days. It has found limited applications in portable X-ray devices and in some medical treatments. Therefore, while elemental thulium in its natural state is not radioactive, specific artificial isotopes of thulium are radioactive and require appropriate safety protocols.
Flammability
Thulium metal in bulk form is not easily flammable at room temperature. However, finely divided thulium powder or dust is combustible and can ignite spontaneously in air at elevated temperatures. Like many finely divided metal powders, thulium dust can pose an explosion hazard when suspended in air, similar to hazards associated with coal dust or flour dust.
Characteristic Chemical Reaction
A characteristic reaction demonstrating thulium’s metallic and reducing properties is its direct combination with halogens (Group 17 elements) to form thulium(III) halides. For instance, thulium reacts vigorously with chlorine gas (Cl₂) to form thulium(III) chloride ($ \text{TmCl}_3 $). This reaction highlights its strong tendency to achieve the +3 oxidation state.
The chemical equation for the reaction with chlorine is: $2\text{Tm(s)} + 3\text{Cl}_2\text{(g)} \rightarrow 2\text{TmCl}_3\text{(s)}$