By Chris Deziel
Updated Mar 24, 2022
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Organic compounds—those that form the chemistry of life—are defined by the presence of carbon. As the sixth most abundant element in the universe and sixth on the periodic table, carbon’s unique electronic structure gives it an unrivaled versatility. Its two inner‑shell electrons and four outer‑shell electrons allow it to form four strong covalent bonds, a property that enables the assembly of vast, stable molecules even in aqueous environments. This makes carbon indispensable not only for Earth’s biology but also for any life that might exist elsewhere.
Carbon’s four valence electrons let it form diverse, robust covalent bonds, creating molecules that stay intact in water. Nearly 10 million unique carbon compounds have been catalogued, underpinning all living systems.
Atoms strive for an octet—eight electrons in their outer shell—through ionic or covalent bonding. Carbon, with its four valence electrons, can both donate and accept electrons, forming up to four covalent bonds simultaneously. This flexibility is illustrated by methane (CH₄), where each hydrogen shares one electron pair with carbon, fulfilling both atoms’ octet requirements.
When two carbon atoms share a single electron pair, they create a strong bond with three remaining bonding sites each. Adding more carbon atoms expands the network rapidly, yielding linear chains, rings, or complex polycyclic structures. The combinatorial possibilities are vast, explaining why chemists have identified almost 10 million distinct carbon-based molecules. Among the most vital for life are carbohydrates, lipids, proteins, and nucleic acids—most famously, DNA.
Silicon, just below carbon in the periodic table, is 135 times more abundant on Earth and also has four outer‑shell electrons. However, silicon’s valence electrons reside in the third orbital, which can accommodate up to 18 electrons, resulting in longer, weaker bonds. Consequently, silicon–silicon bonds are less robust than carbon–carbon bonds at life‑friendly temperatures, and silicon‑based compounds often dissolve or decompose in water. Additionally, silicon dioxide is a solid, so any silicon‑based metabolism would have to expel solid waste—an unlikely evolutionary outcome. These factors, combined with the prevalence of oxygen and the necessity of gaseous CO₂ for energy cycles, favor carbon as the chemistry of life.
