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Name: Dann
Status: student
Grade: 12
Country: India
Date: April 2009


Question:
Why doesn't silicon form compounds like carbon? I ask this because silicon also has valence of 4. But I rarely find silicon compounds contrary to carbon compounds. There is a separate branch of chemistry for carbon. Elements like sulphur, titanium, vanadium, manganese, germanium also have valence four. But why do these compounds not form complex compounds like carbon? Is there a possibility of silicon based life, or titanium based life...etc?



Replies:
Silicon will form multiple (4) covalent bonds like carbon. Pure silicon also has a tetrahedral arrangement around each atom like carbon. there are many silicon based rubber and plastic like compounds like carbon. To my knowledge, there is no evidence of silicon based life on this planet. That does not mean it could not exist. (here or elsewhere)

The other substances you mentioned, while they do have a valence of four, are metals and tend to form bonds that are more ionic in nature than covalent. Tin and lead also exist in the plus four states, but again, in ionic bonds.

Sulfur does form eight member rings naturally and could probably by coaxed into forming long chain compounds. You are probably familiar with yellow sulfur (that is the eight member ring sulfur). Get ahold of a sample, melt it and pour the melt into cold water. something very different will be produced. I do not want to tell you because its better to see for yourself and an experiment thatis easy to do.

Vincent Ward


Dann,

While you are correct that Si should form the same type of bonds that C does and should therefore produce compounds analogous to proteins, carbohydrates, etc. there are three major issues that I can imagine with silicon-based life forms.

1) Since Si-Si bonds are weaker than C-C bonds, the Si analogs of proteins and carbohydrates would be very unstable and would therefore be highly mutable.

2) Si-O bonds are more stable and silicone (-Si-O-) molecules are more likely to form than silane (-Si-Si-) molecules. While silicone molecules are the more stable long-chain form formed by Si, even this bond is not quite as strong as the C-C bond. And, because of this, chirality (the "handedness" of molecules such as enzymes, sugars, etc) is not possible (4-different things must be attached to the Si, 2 O's do not count as different). While life need not be chiral-based, we do know that on Earth C-based life are all chiral-based.

3) Lastly, even if Si-Si or Si-O could be fashioned into complex life-forming compounds (and by the way, some diatoms do incorporate Si into their shells), inspiration -the intake of O2 and subsequent reduction of O2 to O(2-) and expiration -the expulsion of CO2 as waste, would be a problem for Si-based life forms because SiO2 rapidly becomes silica which is a solid. Thus, biological activity that produces waste would mean that a Si-based life-form would need to expel solid silica with every "breath".

So, on the one hand, Si-based life forms might be very unstable compared to C-based, and on the other hand, Si-based life forms would have to have a mechanism for excreting solid waste with every breath which would slow down most metabolic pathways and correcting for those instabilities become slowed.

Finally, consider that Earth is more abundant in Si than C, but natural evolution chose to use C instead of Si, and it really does not look good for Si-based life.

Greg (Roberto Gregorius)


Dann,

Carbon-based chemistry, organic chemistry, relies on two characteristics of the carbon atom. (1) Carbon has 4 electrons in its valence shell and readily forms single, double and even triple bonds with neighboring atoms. (2) Carbon readily forms bonds with other carbon atoms to form rings (such as benzene) and chains (such as the alkanes, like propane, octane and dodecane). Silicon also has 4 valence electrons and can form single bonds and sometimes double bonds. The difference between Si and C is that C is so much smaller. My copy of the periodic table lists C radius = 0.91 angstroms (1 x 10^-1m) while silicon radius = 1.46 angstroms. The periodic law suggests that C and Si have similar chemistries -- they in fact do. Most of the differences are due to Si's larger size and intrinsically lower attraction for valence electrons. Some chemists, like me, specialized in non-carbon chemistry; we call ourselves inorganic chemists. You also asked whether silicon could form the basis of a life system just as carbon does on earth. In theory, yes. Research and time will tell.

Warren Young


Simply having a "valence" number, whether it is 1,2,3,4,5,or 6 does not fully define the properties of the element. Remember, the elements do not "know" what "valence" they have. We humans have assigned that number, not the substance. Many other factors, such as the electronegativity, the other elements with which a particular element might react, and many other factors define the chemistry of a particular element.

Whether there could be life forms based on other elements is an unanswered question. No one knows.

Vince Calder


Dann,

I love this question, it is probably before your time but I remember an episode from the original Star Trek series where Capt. Kirk meets a Si based life form..... it was a sort of rock monster spewing hot lava, you should look it up!

C is in the same group as Si but their chemistry varies hugely. The main reason for this is the small size of the C atom vs the Si atom. This has two effects:

First, Si has large, diffuse P orbitals, which do not overlap well with other p orbitals in Pi bonding, so Si does not tend to form multiple bonds easily. This means that Si, with a normal valency of 4 needs to form four separate single bonds giving giant covalent structures to most of its compounds. C, on the other hand, can form multiple bonds and can easily form small discrete molecules with atoms of lower valencies, such as CO2 with oxygen. Si instead forms the giant structures of quartz and the myriad of silicates.

Secondly the Si-Si bond is considerably weaker than the C-C bond (and multiple Si-Si bonds do not exist, as far as I know) so the formation of Si-Si chains is unlikely, unlike for C.

So C forms countless different compounds with other non metals and itself, where as silicon does not tend to form the same variety of intricate molecules and polymers. However, the different forms of the silicates and their minerals constitute a huge branch of chemistry.

The other "4 valency" elements you listed do not form covalent bonds in the same way as Si and C, due to low electronegativities. So cannot compete with the vast array of C chemistry.

Could Si lifeforms exist? I think it is unlikely, think of how our organic systems work, think of the role of CO2 and small organic molecules. Could Si offer this? Certainly not at the temperatures we see C life. But maybe a silicate based rock monster is possible, at huge temperatures???

Best wishes.

Tom Collins



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