Ask A Scientist© General Science Archive

name       LRB
age        15

Question - We all know that Carbon-based life exists. Why are so many
people talking about Silicon-based life? Why not Germanium or Lead-based
life? Lead and Germanium are in the same column as carbon and silicon and
may therefore have similar properties.
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LRB -

Carbon is an important element in living organisms because of the electron
structure of its outer ring. If you study the theory of molecule formation, it
becomes apparent that carbon has the potential for combining with many other
elements and in a variety of ways... e.g., carbon with one oxygen or with two
oxygen... CO or CO2.

Because silicon is in the same family on the periodic table, it has similar
characteristics (though it has a larger mass). This combined with the fact
that a large amount of silicon is found in the parts of the universe we have
sampled leads some to consider the possibility that life may have evolved 
based
on silicon in other locations. Remember the sand people in Star Wars? That
was the science part of science fiction.

LEK
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As you increase the atomic number and corresponding size of an atom that
will form four bonds i.e., Si, you will get, in general, a more brittle
group of molecules.

PF
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As you get up into heavier elements in the periodic table, their abundance
tends to go down. The likelihood of a planet with sufficient
concentrations of germanium for a life form is small.

Donald Yee Ph.D.
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The element carbon possesses a unique property not possessed by any other
element.

It has the unique ability to form bonds with itself, essentially without
limit. Hence it alone is able to form the complex chemical compounds
required for "life" as we know it.

The only other element that "might" be able to form bonds with itself is
silicon, because lies below carbon in the periodic table. However, to date,
compounds containing even a single Si--Si bond are not very stable except in
very controlled environments.

V. Calder
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It begins with the guess that life elsewhere will depend on the availability
of large molecules that can carry out specific functions (such as movement,
information storage, energy conversion and storage) for a living organism.
In earth's life forms, these molecules are carbon-based.

Chemically, nothing else forms the variety of large molecules that carbon
does. Silicon is the closest. Silicon forms the basis of several types of
macromolecules. Not as many or as varied as carbon, but more than any other
element. Tin and lead are not nearly such good candidates.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
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