of the lanthanides. Most ytterbium compounds are found in the +3 oxidation state, and its salts in this oxidation state are nearly colorless. Like europium, samarium, and thulium, the trihalides of ytterbium can be reduced to the dihalides by hydrogen, zinc dust, or by the addition of metallic ytterbium. The +2 oxidation state occurs only in solid compounds and reacts in some ways similarly to the alkaline earth metal compounds; for example, ytterbium(II) oxide (YbO) shows the same structure as calcium oxide (CaO). Halides Ytterbium forms both dihalides and trihalides with the halogens fluorine, chlorine, bromine, and iodine. The dihalides are susceptible to oxidation to the trihalides at room temperature and disproportionate to the trihalides and metallic ytterbium at high temperature: 3 YbX2 → 2 YbX3 + Yb (X = F, Cl, Br, I) Some ytterbium halides are used as reagents in organic synthesis. For example, ytterbium(III) chloride (YbCl3) is a Lewis acid and can be used as a catalyst in the Aldol and Diels–Alder reactions. Ytterbium(II) iodide (YbI2) may be used, like samarium(II) iodide, as a reducing agent for coupling reactions. Ytterbium(III) fluoride (YbF3) is used as an inert and non-toxic tooth filling as it continuously releases fluoride ions, which are good for dental health, and is also a good X-ray contrast agent. Oxides Ytterbium reacts with oxygen to form ytterbium(III) oxide (Yb2O3), which crystallizes in the "rare-earth C-type sesquioxide" structure which is related to the fluorite structure with one quarter of the anions removed, leading to ytterbium atoms in two different six coordinate (non-octahedral) environments. Ytterbium(III) oxide can be reduced to ytterbium(II) oxide (YbO) with elemental ytterbium, which crystallizes in the same structure as sodium chloride. Borides Ytterbium dodecaboride (YbB12) is a crystalline material that has been studied to understand various electronic and structural properties of many chemically related substances. It is a Kondo insulator. It is a quantum material; under normal conditions, the interior of the bulk crystal is an insulator whereas the surface is highly conductive. Among the rare earth elements, ytterbium is one of the few that can form a stable dodecaboride, a property attributed to its comparatively small atomic radius. History Ytterbium was discovered by the Swiss chemist Jean Charles Galissard de Marignac in the year 1878. While examining samples of gadolinite, Marignac found a new component in the earth then known as erbia, and he named it ytterbia, for Ytterby, the Swedish village