Silicon °C respectively are the second-highest among all the metalloids and nonmetals, being only surpassed by boron. Silicon is the eighth most common element in the universe by mass, but very rarely occurs as the pure element in the Earth's crust. It is most widely distributed in dusts, sands, planetoids, and planets as various forms of silicon dioxide (silica) or silicates. More than 90% of the Earth's crust is composed of silicate minerals, making silicon the second most abundant element in the Earth's crust (about 28% by mass) after oxygen. Most silicon is used commercially without being separated, and often with
Antimony in the sulfide stibnite (SbS) which is the predominant ore mineral. Antimony compounds are often classified according to their oxidation state: Sb(III) and Sb(V). The +5 oxidation state is more stable. Antimony trioxide is formed when antimony is burnt in air. In the gas phase, the molecule of the compound is , but it polymerizes upon condensing. Antimony pentoxide () can be formed only by oxidation with concentrated nitric acid. Antimony also forms a mixed-valence oxide, antimony tetroxide (), which features both Sb(III) and Sb(V). Unlike oxides of phosphorus and arsenic, these oxides are amphoteric, do not form well-defined oxoacids,