Carbon forms the backbone of biology for all life on Earth. Complex molecules are made up of carbon bonded with other elements, especially oxygen, hydrogen and nitrogen. It is these elements that living organisms need, among others, and carbon is able to bond with all of these because of its four valence electrons. Since no life has been observed that is not carbon-based, it is sometimes assumed in astrobiology that life elsewhere in the universe will also be carbon-based. This assumption is referred to by critics as carbon chauvinism, as it may be possible for life to form that is not based on carbon. General Name, Symbol, Number carbon, C, 6 Chemical series nonmetals Group, Period, Block 14, 2, p Appearance black (graphite) colorless (diamond) Atomic mass 12. ... General Name, Symbol, Number oxygen, O, 8 Chemical series Chalcogens Group, Period, Block 16, 2, p Appearance colorless Atomic mass 15. ... General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... This article does not cite any references or sources. ... In chemistry, valence electrons are the electrons contained in the valence shell of an atom, and which are likely to participate in a chemical reaction through bonding with other atoms or molecules. ... Astrobiology is an interdisciplinary field, combining aspects of astronomy, biology and geology, which considers the question of whether extraterrestrial life exists and if so, then what are its origin, distribution, and evolution. ... Carbon chauvinism is the viewpoint in xenobiology that carbon is necessarily the basis of all life on other planets, as carbons chemical and thermodynamic properties render it far superior to all other elements. ... Alternative biochemistry collectively refers to an assortment of astrobiology theories and hypotheses in which life is based on chemical systems other than those used by currently known forms of life. ...

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