A binary compound of oxygen with another element. Oxides have been prepared for essentially all the elements, with the exception of the noble gases. Often, several different oxides of a given element can be prepared; a number exist naturally in the Earth's crust and atmosphere: silicon dioxide (SiO₂) in quartz; aluminum oxide (Al₂O₃) in corundum; iron oxide (Fe₂O₃) in hematite; carbon dioxide (CO₂) gas; and water (H₂O).