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Shine on you crazy diamond: why humans are carbon-based lifeforms

The views expressed are those of the author and are not necessarily those of Scientific American.

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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.

As well as being the main element in organic matter, carbon atoms are the only element in both graphite and diamond. Image credit below.

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.

Carbon forming four bonds with four hydrogen atoms.

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:

Each carbon has formed four bonds (four lines coming out of each C) but two of the bonds are with the same molecule.

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:

Two spherical orbitals overlap and share electrons and electron-space. The carbon nucleus is in the centre of each sphere.

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:

The first bond is shown by the line surrounded by the fuzzy green plane. Image credit below

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.

Silicon dioxide (sand) on the left, carbon dioxide on the right

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.

Credit link for image 1

Credit link for image 5

S.E. Gould About the Author: A biochemist with a love of microbiology, the Lab Rat enjoys exploring, reading about and writing about bacteria. Having finally managed to tear herself away from university, she now works for a small company in Cambridge where she turns data into manageable words and awesome graphs. Follow on Twitter @labratting.

The views expressed are those of the author and are not necessarily those of Scientific American.

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  1. 1. alan6302 3:12 pm 11/12/2012

    The crop formations has a message that claims to be from a silicon life form that is 4 feet tall. That is someone’s interpretation. Researchers are now working on carbon based electronics.

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  2. 2. SteveO 6:05 pm 11/12/2012

    *sigh* One would think that intelligent, interstellar, 4 foot tall silicon life forms could think of a better way to communicate with their carbon brethren then squashing down their fields. What would Occham choose between people playing a prank on their farming friends and four foot silicon based life forms attempting communication, in all violation of known chemistry?

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  3. 3. alan6302 8:45 am 11/13/2012

    On the other hand , the starchild DNA project indicate that the DNA is very strange. Starchild is a very strange 1000 yr old skull that is almost completely ignored. No proof of silicon base in that skull yet.

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  4. 4. S.E. Gould in reply to S.E. Gould 9:03 am 11/13/2012

    Thanks for the comments! Carbon-based electronics sounds pretty fascinating. I’m pretty sure it is well established now that crop circles are made by bored blokes in Wiltshire, and while it is technically vandalism the kind of large-scale patterns and shapes that can be made by three people and a plank at midnight is quite amazing.

    The starchild skull is fascinating, but fairly conclusively human – and even if it were not, it is beyond unlikely that silicon-based lifeforms would survive on earth (for a start, if built anything remotely like a human they’d be breathing out sand!)

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