Introduction to Lanthanum
Lanthanum, symbolized as La, is the first element in the lanthanide series, a group of elements known as rare earth metals. It is a soft, silvery-white, malleable, and ductile metal. Although categorized as a rare earth metal, lanthanum is relatively abundant in the Earth’s crust, found in minerals such as monazite and bastnäsite. Major global deposits of these minerals are found in countries like China, the United States (historically), and Australia.
Chemical Reactivity of Lanthanum
Lanthanum is a highly reactive metal, particularly when compared to transition metals like iron or copper. Its reactivity stems from its electron configuration, which readily allows it to lose three valence electrons to form a positive ion ($\text{La}^{3+}$).
Reaction with Water
Lanthanum reacts with water, though the vigor of the reaction depends on the water’s temperature. It reacts slowly with cold water and more rapidly with hot water to produce lanthanum hydroxide and hydrogen gas. This behavior is typical of active metals. The general reaction can be represented as:
$2 \text{La} (s) + 6 \text{H}_2\text{O} (l) \rightarrow 2 \text{La}(\text{OH})_3 (aq) + 3 \text{H}_2 (g)$
Reaction with Air
When exposed to air, lanthanum metal quickly tarnishes, losing its silvery luster as it oxidizes to form lanthanum oxide. This process is accelerated in moist air. If heated, lanthanum burns readily in air to form lanthanum(III) oxide. Finely divided lanthanum, such as powder or turnings, can ignite spontaneously in air at room temperature, exhibiting pyrophoric properties. The reaction is:
$4 \text{La} (s) + 3 \text{O}_2 (g) \rightarrow 2 \text{La}_2\text{O}_3 (s)$
Safety Profile of Lanthanum
Understanding the potential hazards associated with any chemical element is crucial for safe handling and applications.
Toxicity Considerations
Pure lanthanum metal is generally considered to have low acute toxicity. However, ingestion of lanthanum compounds or inhalation of lanthanum dust and fumes should be avoided. Exposure can cause irritation to the eyes and respiratory tract. Some lanthanum compounds have been observed to interfere with human metabolism, particularly calcium metabolism, but significant health effects from typical environmental exposure are not well-documented.
Radioactivity
Naturally occurring lanthanum is primarily composed of two isotopes: lanthanum-139 ($\text{La}^{139}$), which is stable and accounts for about 99.9% of natural lanthanum, and lanthanum-138 ($\text{La}^{138}$), which is radioactive with an extremely long half-life of over $10^{11}$ years. Due to the very low abundance and extremely long half-life of $\text{La}^{138}$, natural lanthanum is not considered a significant source of radioactivity in practical applications or environmental contexts.
Flammability
Lanthanum is a flammable metal. In bulk form, it requires heating to ignite, but as a powder, turnings, or fine dust, it is highly flammable and can ignite spontaneously in air (pyrophoric). This characteristic necessitates careful handling and storage, especially when working with finely divided forms of the metal. Fires involving lanthanum should be extinguished with specialized metal fire extinguishers (Class D extinguishers) rather than water, which can react with the metal.
Notable Chemical Reaction Involving Lanthanum
One of the most widely recognized examples of lanthanum’s reactivity is its use in mischmetal, an alloy composed primarily of rare earth elements, with lanthanum being a significant component. Mischmetal is famously used in lighter flints. When struck against a rough surface, such as the striker wheel of a cigarette lighter, small particles of mischmetal are scraped off. These small particles, due to their fine division and the highly reactive nature of the rare earth elements (including lanthanum), immediately oxidize and ignite spontaneously in the air. This rapid oxidation generates sparks hot enough to ignite the lighter’s fuel, such as butane. This property, where a material ignites readily on exposure to air without external heat, is known as pyrophoricity.