User:Double sharp/Terbium

Terbium is a soft, ductile, malleable silvery-white metallic chemical element with symbol Tb and atomic number 65. It is the ninth member of the lanthanide series and is traditionally counted among the rare earths. Terbium is too reactive to be found in nature as a free element, but it is contained in many minerals, including cerite, gadolinite, monazite, xenotime, and euxenite.

Terbium always occurs naturally together with the other rare earth metals. It is one of the less common rare earth elements, making up 1.2 parts per million of the Earth's crust, though this abundance is still comparable to those of molybdenum and tungsten and is about six times that of indium, antimony, and cadmium. It was discovered by Carl Gustaf Mosander: after his successful 1841 splitting of cerium into new elements, including lanthanum and "didymium" (later found to be a mixture of praseodymium and neodymium), he turned his attention to yttrium in 1843 and likewise split out two new metals. Yttrium had been named for the Ytterby quarry in Sweden where it was originally discovered in gadolinite; since it had now been split into three parts, Mosander likewise split the name of Ytterby into three parts to name his new elements terbium and erbium. Due to later confusion, the names of terbium and erbium were later switched from Mosander's original. Even Mosander's terbium was impure; in 1878 Jean Charles Galissard de Marignac seperated out the new element gadolinium from Mosander's terbium. Highly pure terbium was not obtained until the advent of ion exchange techniques, due to the extreme similarity of the lanthanides hindering attempts at their separation.

Like most rare earth elements, terbium most readily forms the +3 oxidation state, which is the only stable state in aqueous solution, although the +4 oxidation state is known in some solid compounds. Aqueous Tb3+ ions are very pale pink and almost colourless; the reddish-brown Tb4+ ions are quickly reduced by water to Tb3+.

Terbium is used to dope calcium fluoride, calcium tungstate and strontium molybdate, materials that are used in solid-state devices, and as a crystal stabilizer of fuel cells which operate at elevated temperatures. As a component of Terfenol-D (an alloy that expands and contracts when exposed to magnetic fields more than any other alloy), terbium is of use in actuators, in naval sonar systems and in sensors. Most of the world's terbium supply is used in green phosphors. Terbium oxide is in fluorescent lamps and TV tubes. Terbium green phosphors are combined with divalent europium blue phosphors and trivalent europium red phosphors to provide "trichromatic" lighting technology, a high-efficiency white light used for standard illumination in indoor lighting.

Physical
Terbium is the ninth member of the lanthanide series. In the periodic table, it appears between the lanthanides gadolinium to its left and dysprosium to its right, and above the actinide berkelium. It is malleable, ductile and soft enough to be cut with a knife. It is relatively stable in air compared to the earlier, more reactive lanthanides in the first half of the lanthanide series. The 65 electrons of a terbium atom are arranged in the electron configuration [Xe]4f96s2; normally, only three electrons can be removed before the nuclear charge becomes too great to allow further ionization, but in the case of terbium, the stability of the half-filled [Xe]4f7 configuration allows further ionization of a fourth electron in the presence of very strong oxidizing agents such as fluorine gas.

Terbium exists in two crystal allotropes with a transformation temperature of 1289 °C between them.