OXIDE, in chemistry, a binary compound of oxygen and another element. Oxides are the most plentiful and characteristic components of the earth's hydrosphere and lithosphere ; the former of these envelopes consists essentially of water, the commoner oxide of hydrogen, and the latter largely consists of simple and complex oxides. The second type of oxide is the more abundant and includes such important rock-forming materials as carbonates, silicates, aluminates, and aluminosilicates. The ordinary ingre dients of soil, subsoil, and rocks, including sand, clay, marl, chalk, limestone, dolomite, mica, shale and slate are composed of oxides in either anhydrous or hydrated forms.
Oxygen can be made to combine directly with most elements, although sometimes it will not do so in the entire absence of water vapour (see DRYNESS, CHEMICAL) ; in a few cases in which direct methods are unavailing, as, for example, in the case of noble metals such as gold or platinum which remain unaffected in air even at very high temperatures, indirect methods can be used and oxides can be prepared from the salts of the metals. Hitherto,
however, it has been found impossible to prepare oxides of bro mine or of the inert gases helium, neon, argon, etc. ; fluorine was (until 1927) regarded as being in this category, but it has now been made to yield an oxide indirectly. (See FLUORINE.) Many elements form a series of several oxides ; thus, nitrogen yields five: N20, NO, N203, NO2 (N204 at lower temperatures), and In general, the acidic character of the oxide increases with increase in the oxygen content. For purposes of classification, it is usual to assign a typical oxide to each element; usually this oxide is the highest having acidic or basic properties and is related to the position of the element in the periodic classification (see