Chemical Reactivity of Terbium
Terbium (Tb), a member of the lanthanide series, is a silvery-white, malleable, and ductile metal. Its chemical reactivity is characteristic of the rare-earth elements, exhibiting a strong tendency to lose electrons and form positive ions, typically with a +3 oxidation state.
Reaction with Air
Terbium metal is moderately reactive with atmospheric gases. It tarnishes slowly in dry air at room temperature, forming a protective layer of terbium oxide (Tb₂O₃). However, when exposed to moist air, the tarnishing process accelerates. Upon heating in air, terbium readily burns to form a mixed oxide, primarily terb₄O₇. When in powdered form, terbium can be pyrophoric, meaning it ignites spontaneously in air without an external heat source. This pyrophoric nature necessitates careful handling of finely divided terbium.
Reaction with Water
Terbium reacts with water, although its reactivity depends on the water’s temperature. It reacts slowly with cold water to produce terbium hydroxide and hydrogen gas. The reaction becomes significantly more vigorous with hot water or steam, leading to a faster evolution of hydrogen gas and the formation of terbium hydroxide.
$$2Tb(s) + 6H_2O(l) \rightarrow 2Tb(OH)_3(aq) + 3H_2(g)$$
Toxicity
Terbium compounds are generally considered to have low to moderate toxicity. While elemental terbium dust and its soluble compounds can be irritating to the eyes and skin, and inhalation may cause respiratory irritation, there is limited extensive research on long-term toxicological effects in humans. As with many heavy metals, ingestion of significant quantities is not recommended. Specific occupational safety precautions are advised when handling terbium and its compounds.
Radioactivity
Terbium is not radioactive in its naturally occurring forms. All of its naturally occurring isotopes, such as terbium-159 ($^{159}Tb$), are stable. Several synthetic radioactive isotopes of terbium have been produced in laboratories for scientific research and medical applications, but these are not found naturally and have relatively short half-lives.
Flammability
As a bulk metal, terbium is not considered highly flammable under normal conditions. However, when heated to high temperatures, it will combust. As previously mentioned, in finely divided powder form, terbium is pyrophoric and can ignite spontaneously in air, posing a significant fire hazard. Proper storage in inert atmospheres (such as argon) or under mineral oil is often employed for powdered terbium to prevent spontaneous combustion.
Chemical Reaction Example: Green Phosphors
One of the most notable chemical applications of terbium involves its use as an activator in green phosphors, a technology once ubiquitous in households globally, particularly in cathode ray tube (CRT) television screens and compact fluorescent lamps (CFLs). In these applications, compounds containing terbium ions, specifically the Tb(III) ion, are incorporated into a host material (e.g., yttrium aluminum garnet doped with terbium, YAG:Tb, or yttrium orthosilicate, YSO:Tb). When these materials are excited by an electron beam (in CRTs) or ultraviolet light (in CFLs), the Tb(III) ions absorb energy and subsequently emit light in the characteristic vibrant green region of the visible spectrum. This process of luminescence, based on the specific electronic transitions within the terbium ion, is crucial for creating the green component of full-color displays.