" Silicon-based life "
Every cell of all living organisms on the earth cannot do without the three elements of carbon, hydrogen and oxygen. Among them, carbon is the skeleton of organic molecules, so we call organisms on the earth carbon-based life. Biology and chemists generally believe that if there is a living body somewhere outside the earth, it must be carbon-based like us.
Imagination diagram of alien organisms
1891, based on the chemical similarity between silicon and carbon, German astrophysicist Julius Scheiner took the lead in proposing the idea that silicon replaces carbon in the elements of life. Some science fiction writers and science fiction fans also feel that the vast universe is vast and boundless, and the natural conditions of the alien planet are different from the earth. The living organisms it fosters may not be based on carbon as its skeleton, but may be silicon. This creature is called "silicon-based life."
Alien Silicon
Why is it silicon rather than other elements?
If you still remember the periodic table of elements, it is not difficult to memorize the first twenty elements: hydrogen, helium, lithium, beryllium, boron, carbon, nitrogen, oxygen, fluorine, neon sodium, magnesium, aluminum, silicon, phosphorus, thiochloride, argon, potassium calcium... Among them, carbon (C) ranks 6th, and silicon (Si) ranks 14th.
Silicon is the second highest content element in the earth's crust. It accounts for 27.2% of the total mass of the earth's crust by mass; at the same time, more than 90% of the earth's crust are silicate minerals, which dominate the absolute dominance; and in the universe, by mass, the content of silicon ranks 8th, which is also quite considerable.
Silicon is close to carbon
Silicon is not only rich in content, but also has similar chemical properties to carbon. In the main group IV of the periodic table, silicon is below the carbon, and their outermost layer has 4 valence electrons; it can be combined with hydrogen to form organic ane; pure silicon is like diamond, diamond cubic crystal structure, they are very hard and have a high melting point. So some people naturally believe that since these two elements are so similar, there is rich carbon-based life on the earth, and silicon-based life should also exist.
Silicon-based humans in science fiction movies
Carbon-based advantages
Although the carbon content on the earth's surface is only 1/920 of silicon, the abundance of carbon in Milky Way is 7 times that of silicon. Carbon is formed inside stars, and lighter elements can easily be compressed into heavier carbon through the "triple α process" and the "CNO fusion cycle". In contrast, the production of silicon requires giant star collapse, which is only possible to produce during the supernova explosion , so silicon contains much less carbon in the universe.
Pure carbon can exist on the earth in the form of graphite or diamond. It has good stability and will not easily react chemically with oxygen. Carbon or carbon compounds will only form gaseous oxides - carbon dioxide when they are burned in oxygen.
mentioned in the previous section that there are 4 valence electrons in the outermost periphery of carbon, which are located in the 2s and 2p orbitals respectively, which means that it can form 4 covalent bonds with other atoms , with a covalent radius of 77 pm. Due to the short covalent radius, the carbon covalent bond energy is high and the bonding is stable.
The chemical basis of any life must meet three basic requirements: sufficient chemical diversity; stability and reactivity; and the presence of solvents.
On our earth, living organisms need to participate in multiple chemical substances with different functions. It includes amino acid for the production of protein , sugar and nitrogen-containing bases for the production of nucleic acids, ketoic acid and hydroxyl as core metabolism intermediates, and lipids for the production of cell membrane , etc. In order to combine these chemicals to provide biology functions, living organisms need a set of elements to build various molecules, and a "scaffold" should be established in advance among these molecules.
carbon has become the best choice for this life scaffold due to its unique chemical properties.
Carbon is the backbone of life
Carbon atom can form a long chain through two electrons and adjacent carbon, and at the same time form a large molecule through the combination of the other two electrons and other heteroatoms.In this macromolecule, carbon is the scaffold, and heteroatoms provide abundant chemical activity. Carbon scaffolds provide the ability to make macromolecules, thus making a large number of molecules of different functions. The carbon scaffold is relatively stable, while the heteroatoms bound to carbon provide chemical functions and uniqueness to 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 carbon. Why does not even have a life form based on silicon on earth? We can't even find a piece of pure silicon?
This also needs to start with the silicon atom itself.
The radius of 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 in the 3p orbital, with longer bond lengths and different bond angles that make silicon more reactive than carbon and its covalent bonds are also more unstable.
crystal is solid silica
silicon is easily oxidized to form solid silica, which is the main component of sand. So we can't find pure silicon on Earth, we can only find silica and silicate minerals.
Compared with carbon, nitrogen, oxygen and hydrogen, silicon has stronger positive electrical properties, which will form an electron "sink" in the silicon atom, causing it to polarize stronger with other non-metal bonds, which in turn makes molecules with silicon as the core more susceptible to electrophilic and nucleophilic attacks. Silicon compounds are easier to hydrolyze than carbon, and therefore it is more unstable. In an environment with water, silicon cannot be used as the backbone of macromolecules, but will only exist as heteroatoms in the carbon molecule chain .
Hydrogen and oxygen are the two most abundant elements in the universe, and astronomers have confirmed that water molecules are also widely present in the universe. In such an environment, silicon is either oxidized into a solid, or its molecular chains are broken by water molecules, and chemical diversity is out of the question, nor has the opportunity to develop into the basic substance of life.
Terrible alien life in science fiction movies
Comprehensive and conclusions
Many people think that silicon is very similar to carbon. Through the analysis of its elemental properties, we know that they are a pair of "false twins". Their similarities are superficial and their differences are obvious. Carbon and silicon are almost completely opposite in the external conditions of chemical stability, while the natural conditions for silicon to stabilize the formation of complex organic macromolecules do not exist in the universe.
There are no conditions for the formation of silicon life in the universe
In the universe, carbon is much richer than silicon. Hydrogen, oxygen and water are also widely present, and scientists have not received chemical information in the universe's spectrum to support the existence of silicon-based life. Correspondingly, a large amount of evidence shows that polymer carbon-based chemicals are widely distributed in the universe.
The imagined aliens should also be carbon-based
Even if there are conditions for life to reproduce on extraterrestrial planets, it should first be carbon-based. Silicon is either oxidized into stones and buried underground, or hydrolyzed, or live in the text of science fiction novelists.
