Silicon vs. carbon: why silicon life fails in chemistry but wins in chips

Why silicon looked like carbon’s twin

Every living thing on Earth is carbon-based. Carbon atoms link into chains, rings and branching frameworks, and those frameworks become proteins, DNA, fats and sugars. Silicon is the obvious alternative because it sits directly below carbon in the periodic table and, like carbon, is tetravalent: each atom can form four bonds (Wikipedia, "Silicon").

Life as we know it

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Machines that think

Silicon is also everywhere. It makes up 27.7% of Earth’s crust by mass, second only to oxygen (Royal Society of Chemistry), and it is roughly 925 times more abundant than carbon in the crust (Wikipedia, "Hypothetical types of biochemistry"). So why did life on Earth pick the rarer element?

Who first proposed silicon-based life?

The idea is more than 130 years old. According to a 2020 review, it was first proposed in 1891 by the German astrophysicist Julius Scheiner^↗ (Petkowski, Bains & Seager, 2020). Two years later, the British chemist James Emerson Reynolds pointed out that silicon compounds stay stable at temperatures that would destroy carbon molecules (Darling, Encyclopedia of Science). H. G. Wells took the idea further in 1894, in an essay called “Another Basis for Life”:

“silicon-aluminium organisms – why not silicon-aluminium men at once?”

Science fiction ran with it. Stanley G. Weinbaum’s 1934 story “A Martian Odyssey” includes a silicon-based creature (Wikipedia, "A Martian Odyssey"), Isaac Asimov imagined silicone life on scorching planets in 1962 (Asimov, 1962), and in 1967 Star Trek gave the world the Horta. See every silicon-based life form in Star Trek.

Why silicon biochemistry fails

The problem is not that silicon cannot bond. It bonds too well to the wrong things, and not well enough to itself.

Bond	Silicon (kJ/mol)	Carbon (kJ/mol)	What it means
With oxygen	Si–O: 452	C–O: 358	Silicon grabs oxygen and does not let go
With itself	Si–Si: 222	C–C: 346	Silicon chains are much weaker than carbon chains
With hydrogen	Si–H: 318	C–H: 411	Silicon’s hydrogen compounds are less stable

Bond energies from Petkowski, Bains & Seager (2020) (Petkowski, Bains & Seager, 2020).

Three consequences follow.

1. The oxygen trap. When carbon combines with oxygen, it makes carbon dioxide, a gas that living things can breathe out. When silicon combines with oxygen, it makes silica, a refractory solid rather than a gas^↗: essentially sand and quartz. A silicon organism would choke on its own waste.
2. Weak backbones. Carbon-to-carbon bonds let life build long, complex molecules. Silicon-to-silicon bonds are much weaker, so the equivalent silicon molecules are fragile.
3. Water destroys them. Silanes, silicon’s version of the hydrocarbon chains in living things, are very sensitive to water in the presence of trace alkali (Petkowski, Bains & Seager, 2020). Water is the solvent of every known form of life.

Neil deGrasse Tyson made the same point in 2002, writing that complex molecules based on silicon lack the hardiness to survive ecological stress^↗ (Tyson, 2002). His longer take is on our Neil deGrasse Tyson page.

Could silicon life exist somewhere else?

Researchers have checked the obvious escape routes. Very cold solvents such as liquid nitrogen fail because silicon compounds barely dissolve in them. One environment came out surprisingly well:

“Sulfuric acid, surprisingly, appears to be able to support a much larger diversity of organosilicon chemistry than water.”

Even so, the authors’ overall conclusion was blunt:

“In no environment is a life based primarily around silicon chemistry a plausible option.”

A 2007 National Research Council report on “weird life” did conclude that life is possible in forms different from those on Earth^↗, and it singled out Saturn’s moon Titan, which it said is the locale that is arguably likely to support exotic life^↗ (National Research Council, 2007). But the report’s exotic candidates lean on unusual solvents, not on silicon replacing carbon.

Life already uses silicon, as glass

Silicon is not absent from biology. Diatoms and radiolarians, tiny ocean organisms, build intricate skeletons out of silica, and so do some sponges (Wikipedia, "Silicon"). Rice and horsetails pack their tissues with it; in rice, silica can make up to 10% of the shoot’s dry weight (Luyckx et al., 2017). In every case the organism is carbon-based and uses silicon the way we use glass: as a building material.

In 2016, Frances Arnold’s lab at Caltech went a step further. Starting with an enzyme from a bacterium found in an Icelandic hot spring, the team bred a version that forms carbon-silicon bonds, something that before then No living organism is known to put silicon-carbon bonds together^↗. After three rounds of directed evolution, the enzyme made those bonds 15 times more efficiently than the best catalyst invented by chemists^↗ (Caltech, 2016). The work, published in Science (Kan et al., 2016), shows life can learn silicon chemistry, but the organisms remained carbon-based.

Where silicon won instead: the chip

Silicon’s triumph came from a different direction. In 1954 Texas Instruments introduced silicon transistors (Computer History Museum). In 1959, Robert Noyce’s patent described a practical way to build whole circuits on a single piece of silicon (Computer History Museum). By 1971 the region south of San Francisco had a new nickname, Silicon Valley (Wikipedia, "Silicon Valley").

Every AI system in use today runs on descendants of those chips. Silicon did not become the backbone of life. It became the backbone of computing, and computing has produced something that learns, reasons and talks. That is the literal sense in which today’s AI is silicon-based intelligence.

Carbon vs. silicon at a glance

	Carbon	Silicon
Periodic table	Element 6, group 14	Element 14, group 14
Bonds per atom	Four	Four
Oxide	Carbon dioxide, a gas	Silica, a solid (sand, quartz)
Chains in water	Stable	Break down
Cosmic abundance	About 10× silicon	About one tenth of carbon
Role in life on Earth	The backbone of every molecule	Glass-like skeletons and plant armor
Role in technology	Plastics and fuels	Every computer chip

So is AI a silicon-based life form? The chemistry says silicon cannot build cells. Whether it can build something else we might one day call life is the question on our next page.