Inorganic compounds are those that lack carbon and hydrogen atoms. Inorganic compounds are traditionally viewed as being synthesized by the agency of geological systems. In contrast, organic compounds are found in biological systems. Organic chemists traditionally refer to any molecule containing carbon as an organic compound and by default this means that inorganic chemistry deals with molecules lacking carbon. The 19th century chemist, Berzelius, described inorganic compounds as inanimate, not biological, origin, although many minerals are of biological origin. Biologists may distinguish organic from inorganic compounds in a different way that does not hinge on the presence of a carbon atom. Pools of organic matter, for example, that have been metabolically incorporated into living tissues persist in decomposing tissues, but as molecules become oxidized into the open environment, such as atmospheric CO2, this creates a separate pool of inorganic compounds. The distinction between inorganic and organic compounds is not always clear. Some scientists, for example, view the open environment (i.e., the ecosphere) as an extension of life and from this perspective may consider atmospheric CO2 as an organic compound. The International Union of Pure and Applied Chemistry, an agency widely recognized for defining chemical terms, does not offer definitions of inorganic or organic. Hence, the definition for an inorganic versus an organic compound in a multidisciplinary context spans the division between living (or animate) and non-living (or inanimate) matter and remains open to debate according to the way that one views the world.
Inorganic compounds can be formally defined with reference to what they are not — organic compounds. Organic compounds contain carbon bonds in which at least one carbon atom is covalently linked to an atom of another type (commonly hydrogen, oxygen, or nitrogen). Some carbon-containing compounds are traditionally considered inorganic. When considering inorganic chemistry and life, it is useful to recall that many species in nature are not compounds per se but are ions (e.g., proteins, DNA, and RNA). Sodium, chloride, and phosphate ions are essential for life, as are some inorganic molecules such as carbonic acid, nitrogen, carbon dioxide, water, and oxygen. Aside from these simple ions and molecules, virtually all compounds covered by bioinorganic chemistry contain carbon and can be considered organic or organometallic.
Many compounds that contain carbon are considered inorganic, for example, carbon monoxide, carbon dioxide, carbonates, cyanides, cyanates, carbides, and thyocyanates. In general, however, the workers in these areas are not concerned about strict definitions.
A large class of compounds discussed in inorganic chemistry textbooks are coordination compounds. Examples range from substances that are strictly inorganic, such as [Co(NH3)6]Cl3, to organometallic compounds, such as Fe(C5H5)2, and extending to bioinorganic compounds, such as the hydrogenase enzymes.
Minerals are mainly oxides and sulfides, which are strictly inorganic, although they may be of biological origin. In fact, most of the earth is inorganic. Although the components of Earth's crust are well-elucidated, the processes of mineralization and the composition of the deep mantle remain active areas of investigation, which are covered mainly in geology-oriented venues.
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