<Table> <Tr> <Td> </Td> <Td> This section needs expansion . You can help by adding to it . (August 2015) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This section needs expansion . You can help by adding to it . (August 2015) </Td> </Tr> <P> "What we normally think of as' life' is based on chains of carbon atoms, with a few other atoms, such as nitrogen or phosphorus", per Stephen Hawking in a 2008 lecture, "carbon (...) has the richest chemistry ." The most important characteristics of carbon as a basis for the chemistry of life are, that it has four valence bonds, and that the energy required to make or break a bond is at an appropriate level for building molecules, which are stable and reactive . Carbon atoms bond readily to other carbon atoms; this allows the building of arbitrarily long complex molecules and polymers . </P> <P> There are not many other elements which even appear to be promising candidates for supporting life, for example, processes such as metabolism . The most frequently suggested alternative is silicon . Silicon is in the same group in the Periodic Table of elements, and has four valence bonds, and bonds to itself, generally in the form of crystal lattices rather than long chains . It is considerably more electropositive than carbon . Silicon compounds do not readily recombine into different permutations in a manner that would plausibly support lifelike processes . </P>

Carbon is called the backbone of our living environment