BOILING POINT: 1,800K
DENSITY: 5,244/kg m −3
MOST COMMON IONS: Eu 3+ , Eu 2+
Europium is a metallic element discovered in 1901 in Paris by the French scientist Eugène-Anatole Demarcay. It belongs to a series of elements called lanthanides , or 4f elements, extending from lanthanum (atomic number 57) to lutetium (atomic number 71). These elements have low abundances: Europium occurrence in Earth's crust is only 2.1 ppm (parts per million), that is, 2.1 grams (0.07 ounces) per metric ton, and in seawater, its concentration is as low as 4 × 10 −8 ppm.
As a metal , europium is very reactive so that one usually finds it under its trivalent, triply oxidized form (Eu 3+ ion) in oxides or salts. A divalent form (Eu 2+ ) also displays some stability. Two minerals that contain many of the lanthanide elements, which are separated by liquid-liquid extraction, are commercially important: monazite (found in Australia, Brazil, India, Malaysia, and South Africa) and bastnasite (found in China and the United States).
A very interesting property of the europium ions is their bright red (Eu 3+ ) and bright blue (Eu 2+ ) luminescence. The red luminescence has been instrumental in identifying the element and it is of great practical use today. For instance, the red color seen on computer and television screens derives from red light emitted by a europium-containing phosphor (an inorganic compound with 4–7% Eu 3+ ). The same type of material is used in energy-saving fluorescent lamps: Those displaying a warm light contain Eu 3+ . In medicine, antibodies (molecules generating an antibody response, e.g., certain hormones) labeled with a europium-containing compound react with specific antigens, forming antigen-antibody complexes, and the red luminescence helps to quantify these hormones in biological fluids (e.g., blood, urine).
Kaltsoyannis, Nikolas, and Scott, Peter (1999). The f Elements. New York: Oxford University Press.