Iridium iridium (irid`e?m), metallic chemical element; symbol Ir; at. no. 77; at. wt. 192.22; m.p. about 2,410°C;; b.p. about 4,130°C;; sp. gr. 22.55 at 20°C;; valence +3 or +4. Iridium is a very hard, usually brittle, extremely corrosion-resistant silver-white metal with a face-centered cubic crystalline structure. It falls between platinum and osmium in Group 9 of the periodic table. It is not certain whether osmium or iridium is the most dense element. Iridium is found uncombined in nature as the metal and in combination with osmium and platinum. It is obtained commercially from osmiridium, a byproduct of platinum production. The metal is used in pivot bearings and in scientific and other special equipment, such as surgical tools. It is also used in making chemical crucibles. Iridium is used principally in alloys. An alloy with osmium is used to make fountain-pen nibs. Alloys with platinum are used in heavy-duty electrical contacts. An alloy of 10 parts iridium with 90 parts platinum is used in the international kilogram standard in Paris. Formerly the international meter standard was the distance between two marks on a bar made of that same alloy; it is now based on the wavelength of a line in the spectrum of an isotope of krypton. Iridium is chemically very unreactive. Pure iridium metal is not attacked by acids or acid mixtures, not even by aqua regia, which dissolves gold. Fluorine and chlorine attack it only at a red heat. It is oxidized slowly at high temperatures. It resists attack by fused bases and by most molten metals. Iridium was discovered in 1804 by Smithson Tennant; it is so named because of its various highly colored salts

Scandium from Latin Scandia (Scandinavia), where the first rare earth ore was discovered.

Light aluminium-scandium alloy for aerospace components, additive in Mercury-vapor lamps.[4]

Yttrium after the village of Ytterby, Sweden, where the first rare earth ore was discovered.

Yttrium-aluminium garnet (YAG) laser, yttrium vanadate (YVO4) as host for europium in TV red phosphor, YBCO high-temperature superconductors, yttrium iron garnet (YIG) microwave filters.[4], energy-efficient light bulbs[5] Lanthanum from the Greek "lanthanein", meaning to be hidden. High refractive index glass, flint, hydrogen storage, battery-electrodes, camera lenses, fluid catalytic cracking catalyst for oil refineries

Cerium after the dwarf planet Ceres, named after the Roman goddess of agriculture. Chemical oxidizing agent, polishing powder, yellow colors in glass and ceramics, catalyst for self-cleaning ovens, fluid catalytic cracking catalyst for oil refineries, ferrocerium flints for lighters

Praseodymium from the Greek "prasios", meaning leek-green, and "didymos", meaning twin. Rare-earth magnets, lasers, core material for carbon arc lighting, colorant in glasses and enamels, additive in didymium glass used in welding goggles,[4] ferrocerium firesteel (flint) products. Neodymium from the Greek "neos", meaning new, and "didymos", meaning twin. Rare-earth magnets, lasers, violet colors in glass and ceramics, ceramic capacitors

Promethium after the Titan Prometheus, who brought fire to mortals. Nuclear batteries

Samarium after Vasili Samarsky-Bykhovets, who discovered the rare earth ore . Rare-earth magnets, lasers, neutron capture, masers

Europium after the continent of Europe. Red and blue phosphors, lasers, mercury-vapor lamps, NMR relaxation agent

Gadolinium after Johan Gadolin (1760–1852), to honor his investigation of rare earths. Rare-earth magnets, high refractive index glass or garnets, lasers, X-ray tubes, computer memories, neutron capture, MRI contrast agent, NMR relaxation agent

Terbium after the village of Ytterby, Sweden. Green phosphors, lasers, fluorescent lamps

Dysprosium from the Greek "dysprositos", meaning hard to get. Rare-earth magnets, lasers

Holmium after Stockholm (in Latin, "Holmia", native city of one of its discoverers. Lasers

Erbium after the village of Ytterby, Sweden. Lasers, vanadium steel

Thulium after the mythological northern land of Thule. Portable X-ray machines

Ytterbium after the village of Ytterby, Sweden. Infrared lasers, chemical reducing agent

Lutetium after Lutetia, the city which later became Paris. PET Scan detectors, high refractive index glas