Life and the Elements - Animate Earth: Science, Intuition and Gaia - page 71

The most important elements for life and Gaia are just six: carbon, hydrogen, nitrogen,
oxygen, phosphorus and sulphur, remembered in the trade by the acronym CHNOPS.
Let's look at how Bohr's model helps us to understand the personalities of these six
chemical beings. Carbon has a full inner orbit with two electrons, but the next and out-
ermost orbit is incomplete, with only four (Figure 16) . Thus to find completion, carbon
needs another four electrons, and finds them by sharing those in the outer orbits of other
atoms, especially other carbons, forming what chemists call covalent bonds. When this
happens, both atoms at last achieve a satisfactory resting state. The fact that carbon has
a need for four electrons makes it a highly cooperative and intensely social chemical
being. It is the solid, reliable Swede of the chemical world, loving nothing better than
to share each electron with a fellow carbon atom, which means that it can link up with
four neighbouring carbons to make large chains, rings and chains of rings in which mul-
titudes of carbon atoms and associated oxygen, nitrogen, phosphorus and other atoms
find collective fulfilment in the huge, often complexly convoluted molecules of life such
as sugars, proteins and DNA. These linkages amongst carbon atoms are the basis of life
as we know it; without them Gaia could not exist, and our planet would be as devoid of
living beings as is our nearest neighbour, the moon.
The essence of carbon is centrality. It occupies a pivotal place in the community of
elements by virtue of its half-filled outer electron shell ( Figure 16 ) , and it also occupies
a central place in the workings of Gaia: atmospheric carbon dioxide and methane help
to set the global temperature; dissolved carbon compounds regulate the acidity of the
oceans; and, as we shall see, the burial of organic carbon helps to regulate the oxygen
content of the air. The key point about global temperature is this: any gas molecule in
the air that contains two or more atoms delays the escape to space of heat given off by
the Earth after it has been warmed by the light of the sun. Carbon is present in at least
two such greenhouse gases : carbon dioxide (CO 2 , (Figure 17) and methane (CH 4 ). Water
vapour (H 2 O) is another potent greenhouse gas. The addition of these and other green-
house gases to the air raises global temperature; their removal decreases it.