Conditions for Life: Other Elements In addition to water, life requires other elements that have both structural and metabolic roles. As will be seen, some of these form part of the various "vitamins" that human beings need to survive - others are known as "trace elements" and they are equally important for our well-being. The "biogenic elements" are: atomic carbon (C) which comes mainly from Carbon Dioxide (CO2); atomic hydrogen (H) which can be generated from water (H2O) and molecular Hydrogen (H2); atomic oxygen (O) from water (H2O); atomic nitrogen (N) from molecular Nitrogen (N2); atomic sulphur (S) from hydrogen sulphide (H2S); and atomic phosphor (P) obtained from the phosphatic anion (PO43-); together with monovalent and divalent metals like sodium (Na), potassium (K), magnesium (Mg), calcium (Ca), manganese (Mn), iron (Fe), cobalt (Co), copper (Cu) and zinc (Zn); as well as anions like that of chlorine Cl-. Interestingly enough, most of these elements are the ones that are most abundant in the Universe. Also the molecules used to obtain them by the living systems are quite common. So it is reasonable to conclude that life is made of components that are easily available in the environment. Important properties of the biogenic elements Some properties of these elements seem to be important for the building blocks of living organisms. The most abundant correspond to atoms of small size, thus producing more stable chemical bonds. Carbon, oxygen and nitrogen (C, O and N) can form double and triple bonds which give them very peculiar properties. Carbon (C) can combine with oxygen (O) producing carbon dioxide (CO2) which is an important component of the atmosphere (and a greenhouse gas). This property is not shared by silicon (Si), a neighbour of carbon in the periodic table of elements and which otherwise shares many properties with carbon. This could be one of the reasons for the selection of carbon as the basis for life, as we know it.