Life as we know it on Earth is virtually all based on carbon which is referred to as "organic" as in organic chemistry. In science fiction sometimes lifeforms are referred to as "silicon based". This is because on the periodic table of elements silicon and carbon are both in the same group or family (the vertical row).
Just like silicon, carbon has four valance electrons in its outermost shell. This allows it to form long chains with other carbon atoms. Compared to silicon, carbon has a good balance of both forming stable molecules and reactive molecules. Carbon also forms more complex molecules more easily and with less energy than silicon.
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