This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Previous posts in the Chemistry series: Hydrogen-bonds, van der Waals forces, metallic bonding, ionic bonds
Everything on earth is made up of combinations of different elements - all of which can be found on the periodic table. Considering that the periodic table contains 118 elements it seems a pity that organic life tends to feature only five or six of those elements in any vast quantities. The main one being carbon.
It would be impossible for life on earth to exist without carbon. Carbon is the main component of sugars, proteins, fats, DNA, muscle tissue, pretty much everything in your body. The reason carbon is so special is down to the electron configuration of the individual atoms. Electrons exist in concentric 'shells' around the central nucleus and carbon has four electrons in its outermost shell. As the most stable thing for an atom to have is eight electrons, this means that each carbon can form four bonds with surrounding atoms.
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Each bond in the above molecule is formed by the sharing of two electrons; one from the carbon and one from the hydrogen. The ability to form four bonds isn't restricted to carbon though, it's a property of every atom with four outer electrons, including silicon, tin and lead. What's special about carbon, and the reason that silicon-based lifeforms are restricted to science fiction (and lead-based lifeforms are hardly ever mentioned) is that it can form double-bonds which share more than one electron with another atom, as shown below:
Why is carbon able to do this while silicon can't? Although the bonds are all drawn as straight lines in the diagram above, in real life not all bonds are equal. The double bond consists of two different types of bond. Each bond is made up of two electron orbitals (one from each atom) which have overlapped. The easiest way to think of an orbital without getting into some serious physics is as a blurry sort of zone in which a fast-moving electron is most likely to be whizzing about. When two orbitals overlap, you have double the space which two electrons can whiz around in.
The single bond is formed by two circular orbitals overlapping and surrounding both atoms:
The second bond is formed slightly differently. The electrons that form these bonds are not in a spherical orbital around the nucleus, they are in oval-shaped orbitals that protrude above and below the nucleus. When they overlap the bond forms above and below the first bond, as shown in the diagram:
So why can carbon and not silicon manage this double-bond trick? The answer lies in the size. Carbon is the smallest of all the atoms with four outermost electrons, which means that the electrons in the above-and-below orbitals are close enough to overlap and form that second bond. For silicon however, there are more electron orbitals in the way, the entire atom is bigger, and it is almost impossible for the outer orbitals to get close enough to form a double bond. This is why carbon dioxide is is a small gaseous molecule consisting of two oxygens both forming a double bond with a single carbon while silicon dioxide is a massive behemoth of a molecule made of huge numbers of alternating oxygen and silicon atoms and is more commonly known as sand.
You can just about get silicon-silicon double bonds if you try hard, but they are fairly unstable and will take any chance they can to loose that double-bond in favour of forming another single one. Carbon-carbon double bonds on the other hand form naturally and easily, and are crucial for every living organism on earth. If there were to be silicon-based lifeforms, the sheer chemistry of their atoms means that they would have to be built along very different lines to life on earth.
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