Europium

Europium is a chemical element, symbolized as Eu, with atomic number 63. It belongs to the lanthanide series and appears as a silvery-white metal. Among the lanthanides, it is the most chemically reactive, the least dense, and the softest—soft enough to be cut with a knife. It quickly reacts with air, forming a dark oxide coating. Discovered in 1896 and initially given the provisional symbol Σ, it was named in 1901 after the continent of Europe by its discoverer, Eugène-Anatole Demarçay. Like other lanthanides, europium typically takes a +3 oxidation state, but +2 compounds are also common. These +2 compounds are slightly reducing because they tend to oxidize to the more stable +3 state. Europium has no significant biological role and is relatively non-toxic compared to other heavy metals. Most of its applications rely on the phosphorescence of its compounds. It is one of the rarest rare-earth elements on Earth.

Physically, europium is ductile, with a hardness similar to lead, and crystallizes in a body-centered cubic lattice. Along with ytterbium, it has the largest volume per mole of metal among the lanthanoids, which magnetic measurements suggest is because these metals are effectively divalent, while others are trivalent. Chemically, europium behaves broadly like other lanthanoids but is the most reactive. It oxidizes rapidly in air—bulk oxidation of a centimeter-sized sample occurs within days—and its reactivity with water is comparable to calcium, producing europium hydroxide and hydrogen gas. Fresh europium metal rarely looks shiny, even when protected by mineral oil. It ignites in air at 150 to 180 °C, forming europium(III) oxide, and dissolves in dilute sulfuric acid to give pale pink solutions.

Unlike most lanthanides, which almost exclusively form +3 compounds, europium readily forms divalent compounds. The +2 state has a stable 4f⁷ electron configuration due to the half-filled f-shell. Europium(II) and barium(II) are similar in size and coordination number, and their sulfates are both highly insoluble in water. Divalent europium is a mild reducing agent, oxidizing in air to +3. Under anaerobic or geothermal conditions, the divalent form is stable enough to be incorporated into calcium and alkaline earth minerals. This ion-exchange process causes the "negative europium anomaly"—low europium content in minerals like monazite relative to chondritic abundance. Bastnäsite shows less of this anomaly and is the main source of europium today. Easy methods to separate divalent europium from trivalent lanthanides made europium accessible despite its low concentration.

Europium compounds are usually trivalent, with Eu(III) bound by 6–9 oxygen-donating ligands. Its sulfates, nitrates, and chlorides are soluble in water or polar organic solvents. Lipophilic complexes often use acetylacetonate-like ligands, such as EuFOD. Europium metal reacts with all halogens to form trihalides: white fluoride, yellow chloride, gray bromide, and colorless iodide. It also forms dihalides: yellow-green fluoride, colorless chloride (with bright blue fluorescence under UV), colorless bromide, and green iodide. With chalcogens, europium forms stable compounds; heavier chalcogens (S, Se, Te) favor the lower oxidation state. Three oxides exist: europium(II) oxide, europium(III) oxide, and mixed-valence Eu₃O₄. The main chalcogenides are black europium(II) sulfide, selenide, and telluride. Europium(II) sulfide is made by sulfiding the oxide at high temperatures. The main nitride is europium(III) nitride.

Naturally occurring europium has two isotopes: ¹⁵¹Eu and ¹⁵³Eu, in almost equal proportions, with ¹⁵³Eu slightly more abundant (52.2%). While ¹⁵³Eu is stable, ¹⁵¹Eu undergoes alpha decay with a half-life of 4.6×10¹⁸ years, producing about one alpha decay every two minutes per kilogram of natural europium. Besides ¹⁵¹Eu, 39 artificial radioisotopes have been characterized, from ¹³⁰Eu to ¹⁷⁰Eu. The most stable artificial isotopes are ¹⁵⁰Eu (half-life 36.9 years), ¹⁵²Eu (13.516 years), ¹⁵⁴Eu (8.592 years), and ¹⁵⁵Eu (4.742 years). All others have half-lives shorter than 100 days, most under 3 minutes. Europium also has 27 meta states, the most stable being ¹⁵⁰ᵐEu (12.8 hours), ¹⁵²ᵐ¹Eu (9.3116 hours), and ¹⁵²ᵐ⁵Eu (96 minutes).