Europium

Europium is a chemical element with the symbol Eu and atomic number 63. Europium is the most reactive lanthanide by far, having to be stored under an inert fluid to protect it from atmospheric oxygen or moisture. Europium is also the softest lanthanide, as it can be dented with a fingernail and easily cut with a knife. When oxidation is removed a shiny-white metal is visible. Europium was isolated in 1901 and is named after the continent of Europe.[6] Being a typical member of the lanthanide series, europium usually assumes the oxidation state +3, but the oxidation state +2 is also common. All europium compounds with oxidation state +2 are slightly reducing. Europium has no significant biological role and is relatively non-toxic compared to other heavy metals. Most applications of europium exploit the phosphorescence of europium compounds. Europium is one of the rarest of the rare-earth elements on Earth.[7]

Europium is a ductile metal with a hardness similar to that of lead. It crystallizes in a body-centered cubic lattice.[8] Some properties of europium are strongly influenced by its half-filled electron shell. Europium has the second lowest melting point and the lowest density of all lanthanides.[8]

Europium has been claimed to become a superconductor when it is cooled below 1.8 K and compressed to above 80 GPa.[9] However the experimental evidence on which this claim is based has been challenged.[10] If it becomes a superconductor this is believed to occur because europium is divalent in the metallic state,[11] and is converted into the trivalent state by the applied pressure. In the divalent state, the strong local magnetic moment (J = 7/2) suppresses the superconductivity, which is induced by eliminating this local moment (J = 0 in Eu3+).

Europium is the most reactive rare-earth element. It rapidly oxidizes in air, so that bulk oxidation of a centimeter-sized sample occurs within several days.[12] Its reactivity with water is comparable to that of calcium, and the reaction is 2 Eu + 6 H2O → 2 Eu(OH)3 + 3 H2

Because of the high reactivity, samples of solid europium rarely have the shiny appearance of the fresh metal, even when coated with a protective layer of mineral oil.