Carbon dioxide, that small molecule, present in such tiny quantities, is probably the most famous molecule of all. Everybody who reads the papers, I am sure, has heard of its role as a greenhouse gas and the fact that its concentration is increasing, though they may not know the details. Carbon dioxide is also the link between ‘inorganic chemistry and ‘organic chemistry’ through the process of photosynthesis. The ‘true’ organic compounds are the sugars, fats, and proteins made by the plants and used by other living creatures.
The other organic compounds (about 6 million) made in the factories also start here; they are made from compounds found in fossil fuels that were formed from material that was once living. So we can assume that carbon dioxide is the parent molecule to most of the carbon compounds that exist now. The only difference between ‘natural’ and ‘artificial’ compounds is the fact that the latter do not degrade biologically, so are piling up in the environment and causing another environmental crisis.
Textbooks tell us that carbon is unique – it forms all these compounds because it can form four bonds and so form chains, rings, and can form bonds with many elements. It is worth stressing though, that each element is unique and to explain the properties of carbon as a continuum from aluminium to fluorine. The ability to form four bonds is due to the presence of four electrons and while nitrogen can form three and so form long chains too, the lone pairs of electrons on the N atoms repel each other and the chains cannot grow indefinitely. It is also interesting to note that silicon has similar properties but life is not silicon based, except in science fiction. The silicon-silicon bond is weaker than the carbon-carbon bond and in the presence of oxygen, converts more readily to silicon dioxide, than carbon does to carbon dioxide. All carbon compounds eventually form carbon dioxide. This also tells us that chemical properties are more or less invariant through the universe, and it is expected that any other life in the universe will also be carbon based. That is why scientists get so excited when fragments of organic molecules are found in space.
The author works with Centre for Learning, Bengaluru. She can be reached at firstname.lastname@example.org.