2020-10-07

Life on earth is based on carbon. Why is there no silicon-based life in the universe?

By yqqlm yqqlm

The origin of”silicon-based life”

In every cell of all life forms on the earth, carbon, hydrogen and oxygen are inseparable from the three elements, of which carbon is The skeleton of organic molecules, so we call the living things on earth carbon-based life. Biologists and chemists generally believe that if life exists somewhere outside the earth, it must be carbon-based, just like us.

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Imagination of aliens

In 1891, based on the chemical similarity of silicon and carbon, German astrophysicist Julius Scheiner took the lead in proposing silicon The idea of ​​replacing carbon in the element of life. Some sci-fi authors and sci-fi fans also feel that the universe is vast and vast, and the natural conditions of the outer planets are different from those of the earth. The life forms it breeds may not be carbon-based, but may be silicon. This organism is called”silicon-based life.”

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Alien Silicon Man

Why silicon instead of other elements?

If you remember the periodic table, it’s not difficult to recite the first twenty Elements:hydrogen, helium, lithium, beryllium, boron, carbon, nitrogen, oxygen, fluorine, neon, sodium, magnesium, aluminum, silicon, phosphorus, sulfur, chlorine, argon, potassium, calcium… Among them, carbon (C) ranks 6th and silicon (Si) ranks 14th.

Silicon is the second most abundant element in the earth’s crust, and it occupies the earth’s crust by mass 27.2%of the total mass; at the same time, more than 90%of the earth’s crust is silicate minerals, occupying an absolute dominant position; and in the universe, the content of silicon ranks 8th in terms of mass, which is also quite impressive.

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Silicon and carbon are next to each other

Silicon is not only rich in content, it also has similar chemical properties to carbon. In group IV of the periodic table, silicon is below carbon, and their outermost layer has 4 valence electrons; they can combine with hydrogen to form organic alkanes; pure silicon and diamond are both diamond cubic crystals Structure, they are very hard and have a high melting point. So some people naturally think that since these two elements are so similar, there are abundant carbon-based life on the earth, and there should be silicon-based life.

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Silicon people in science fiction movies

Advantages of carbon-based

Although the carbon content on the earth’s surface is only 1/920 of that of silicon, the abundance of carbon in the Milky Way is 7 times that of silicon. Carbon is formed inside stars. Through the”triple alpha process” and the”CNO fusion cycle”, lighter elements can be easily compressed into heavier carbon. In contrast, the production of silicon requires the collapse of giant stars, that is, it has a chance to be produced in the process of supernova explosion. Therefore, the content of silicon in the universe is much less than that of carbon.

Pure carbon can exist on the earth in the form of graphite or diamond. It has very good The stability of carbon dioxide will not easily chemically react with oxygen. Carbon or carbon compounds will only form gaseous oxides-carbon dioxide when burned in oxygen.

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Diamonds can burn in oxygen

As mentioned in the previous section, there are 4 valence electrons on the outermost periphery of carbon, which are located in the 2s and 2p orbitals, which means that it can interact with other atoms It consists of 4 covalent bonds with a covalent radius of 77pm. Due to the short covalent radius, the carbon covalent bond energy is high and the bond is stable.

The chemical basis of any life must beMeet three basic requirements:sufficient chemical diversity; stability and reactivity; the presence of solvents.

On our planet, life forms need many different functions Of chemicals involved. Including amino acids used to make proteins, sugars and nitrogenous bases used to make nucleic acids, keto acids and hydroxyls as core metabolic intermediates, and lipids used to make cell membranes. In order to combine these chemical substances to provide biological functions, living organisms need a set of elements to construct various molecules, and a”scaffold” should be built in these molecules in advance.

Carbon is unique in this life support because of its unique chemical properties Choose.

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Carbon is the skeleton of life

A carbon atom can form a long chain with adjacent carbons through two electrons, and combine with other heteroatoms to form a macromolecule through the other two electrons. In this macromolecule, carbon is the scaffold, and heteroatoms provide abundant chemical activity. Carbon scaffolds provide the ability to make large molecules, so a large number of molecules with different functions can be made. The carbon scaffold is relatively stable, and at the same time, the heteroatoms combined with carbon provide chemical functions and uniqueness for each molecule.

Why can’t silicon?

During the formation of the solar system, the content of silicon on the earth’s surface far exceeds that of carbon. Its atomic structure is similar to that of carbon. Why doesn’t even a silicon-based life form appear on the earth, and we can’t even find a piece of pure silicon?

This also needs to start with the silicon atom itself.

The atomic radius of silicon is larger than that of carbon, and its covalent radius also reaches 117pm. Two of the four peripheral electrons of silicon are located in the 3s orbital and two are located in the 3p orbital. The longer bond length and different bond angles make silicon more active than carbon, and its covalent bonds are also less stable.

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Crystal is solid silica

Silicon is easily oxidized to form solid silicon dioxide, which is the main component of sand. Therefore, we cannot find pure silicon on the earth, but can only find silica and silicate minerals.

Compared with carbon, nitrogen, oxygen and hydrogen, silicon has a stronger positive charge This will form an electron”swap” in the silicon atom, which will cause the bond between it and other non-metals to have stronger polarization, which will make the silicon-core molecules more vulnerable to electrophilic and nucleophilic attacks. Compared with carbon, silicon compounds are more easily hydrolyzed, so it is also less stable. In an environment with water, silicon cannot be used as the skeleton of large molecules, but only as a chain of carbon molecules The presence of heteroatoms.

Hydrogen and oxygen are the two most abundant elements in the universe, and astronomers have confirmed Water molecules are also widespread in the universe. In such an environment, silicon is either oxidized to become a solid, or its molecular chain is broken by water molecules, chemical diversity is impossible to talk about, and there is no chance to develop into a basic substance of life.

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The terrifying alien life in science fiction movies

Comprehension and conclusion

Many people think that silicon and carbon are very similar. Through the analysis of their element properties, we know that they are a pair of”fake twins”. The similarities are Superficially, the difference is obvious. Carbon and silicon are almost completely opposite in terms of the external conditions of chemical stability, while the natural conditions for silicon to stably form complex organic macromolecules do not exist in the universe.

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The universe does not have the conditions for the formation of silicon life

In the universe, carbon is much richer than silicon. Hydrogen, oxygen and water are also widespread. Scientists have not received the chemical information that supports the existence of silicon-based life in the spectrum of the universe. Correspondingly, a large amount of evidence shows that the distribution of high molecular carbon-based chemicals in the universe is very extensive.

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Imaginary aliens, they should also be carbon-based

Even if an extraterrestrial planet has conditions for life to multiply, it should first be carbon-based, and silicon is either oxidized Buried as a rock in the ground, it was either hydrolyzed or lived in the words of science fiction writers.