The Chemical Reactivity of Cerium
Cerium (Ce), element number 58 on the periodic table, is a soft, silvery, ductile metal that belongs to the lanthanide series, often referred to as rare-earth elements. Despite their name, rare-earth elements are relatively abundant in the Earth’s crust; for instance, cerium is about as common as copper. Its chemical behavior is largely determined by its tendency to lose electrons, primarily three, to form a Ce³⁺ ion, though a Ce⁴⁺ state also exists and is important in certain compounds like cerium dioxide.
Reaction with Water
Cerium exhibits moderate reactivity with water. It reacts with cold water, albeit slowly, and more vigorously with hot water, to produce cerium hydroxide and hydrogen gas. This reaction can be represented by the following chemical equation:
2 Ce(s) + 6 H₂O(l) → 2 Ce(OH)₃(aq) + 3 H₂(g)
This behavior places cerium among metals that are more reactive than common transition metals like iron, but less reactive than alkali metals such as sodium.
Reaction with Air
When exposed to air, cerium tarnishes rapidly. It reacts readily with oxygen to form cerium oxides, which typically appear as a dull gray or yellowish layer on its surface. This oxidation process occurs quickly, especially when the metal is in a finely divided state, such as powder or turnings. In fact, powdered cerium is pyrophoric, meaning it can spontaneously ignite in air at room temperature. This characteristic makes careful handling and storage necessary.
Toxicity and Radioactivity
Cerium is generally considered to have low acute toxicity. Exposure to cerium compounds can cause irritation to the eyes, skin, and respiratory tract. If ingested, it can lead to gastrointestinal disturbances. However, it is not classified as a highly toxic heavy metal like lead or mercury.
Regarding radioactivity, naturally occurring cerium is composed of four stable isotopes: Cerium-136, Cerium-138, Cerium-140, and Cerium-142. While Cerium-142 is theoretically predicted to undergo alpha decay with an extremely long half-life, this decay is not typically observed or considered significant for practical purposes. Therefore, for most applications and general understanding, cerium is not regarded as a radioactive element.
Flammability
Cerium is a flammable element, particularly in its finely divided form. As mentioned, powdered cerium is pyrophoric and can ignite spontaneously in air. This property is due to the rapid exothermic reaction of cerium with atmospheric oxygen. When ignited, cerium burns brightly, producing a significant amount of heat and light.
A Famous Chemical Reaction
One prominent example of cerium’s chemical reactivity, specifically its flammability and pyrophoric nature, is its use in flint lighters. Traditional lighter flints are typically an alloy called “Mischmetal,” which is composed of approximately 50% cerium, along with lanthanum and other rare-earth elements. When a steel wheel strikes the Mischmetal flint, small particles of the alloy are abraded. These finely divided cerium-containing particles, due to their pyrophoric nature, ignite spontaneously in the air, producing sparks. These sparks then ignite the lighter fluid or gas, creating a flame. This application is globally ubiquitous, from household lighters in North America to pocket lighters used across Europe and Asia, demonstrating a direct, observable chemical reaction involving cerium.