CARBONIO. – Chemical element with atomic number 6 and atomic weight 12, one of the most important in nature because it is an essential component of living organisms, although it constitutes only 0.1% of the earth’s crust. In the crystalline state it constitutes diamond and graphite; in the amorphous state it is found in the various kinds of coal, both fossil and vegetal; in the combined state it is bonded with oxygen to form the oxide of carbon CO, a toxic gas, and carbon dioxide CO, which, together with the carbonates derived from the latter, constitute the inorganic compounds of this element; finally, it is a constituent of all organic compounds, so that the chemistry of carbon is called organic chemistry.
The carbon cycle in nature is highly remarkable. The green plant fixes and reduces the carbon dioxide contained in the air (algae that dissolved in water), using the energy of solar radiation in a reaction in which water also participates: oxygen is released and the first organic product of synthesis is formed, from which all organized organic matter originates. All living organisms, plants and animals, in the process of respiration burn organic matter and return part of the carbon to the environment in the form of carbon dioxide. The sea acts as a stabilizer of the concentration of carbon dioxide in the atmosphere, since it releases or absorbs it according as its concentration in the atmosphere tends to decrease or increase. The slow combustion of plant and animal residues, emissions of carbon dioxide, combustions carried out by human beings (of wood, coal, and petroleum), and fermentations all contribute to making the carbon cycle more complex: these are all causes that return to the atmosphere the carbon dioxide indispensable for nourishing the crops from which food is obtained for the continually increasing human population.