Silicon is one of the Earth’s two most abundant elements and used in everything from paints and computers to TV screens – yet living organisms have yet to form carbon-silicon bonds between themselves and silicon molecules.
Scientists engineered a bacterium using directed evolution – an approach already widely employed for developing pharmaceuticals, agricultural chemicals and fuels. To do so they began with an electron transport protein found in bacteria called Cytochrome C which acts like a relay between electrons.
Physical
Silicon (Si) is a nonmetallic, tetravalent element found in Group 14 of the periodic table. This element forms covalent chemical bonds with four other atoms to form covalent chemical bonds; part of the carbon family, it belongs to this group and can only form covalent chemical bonds with those formally having four electrons in their valence shells. Lewis dot structure shows this element with four dots surrounding it to indicate this fact.
Silicon is an abundant element that occurs naturally throughout Earth’s crust in various rocks and minerals such as sand, quartz, clays and feldspars, amethyst tourmaline mica asbestos zircon. Ferrosilicon, an essential raw material in steel production also contains silicon as a component. As this element does not occur free in nature but must always combine with oxygen or other elements to form silicates or carbides;
Silicon’s most prominent industrial application is silicones, an umbrella term for polymers such as Silly Putty and Superballs that make up this class of materials. Silicones are used in weatherproofing materials, lubricants, electrical insulation materials and chemicals requiring high temperatures or excellent physical properties – and silicone is no different!
Silicon stands in contrast with carbon in terms of electronegativity; its electronegativity value is much higher, making it more electropositive than carbon and hypervalent elements like halogens and nitrogens. Thus silicon forms bonds easily with these hypervalent elements and halogens while reacting with hydrogen to form carbon-silicon compounds often known as carbides.
Chemical
Silicon is a grey semi-metal with the chemical versatility to form covalent bonds with many elements, similar to carbon. Yet despite being classified in the same group of the periodic table as carbon, its behavior differs considerably.
Carbon and silicon share similarities in terms of chemical properties due to having similar numbers of valence electrons available to bond with other atoms, which in turn determine their reactivity as well as the ability to form various compounds.
silicon, like carbon, possesses four valence electrons that allow it to form covalent bonds with other atoms and molecules as well as organic compounds essential in producing essential materials.
Silicon is another element with high electronegativity values, meaning it draws other atoms very strongly to itself and forms strong bonds with various non-metal elements, essential in creating polymers and other useful substances.
Silicon is an abundant and reactive metal that forms important compounds with oxygen, hydrogen, nitrogen, sulphur and halogens, as well as with other elements such as aluminium. Silicon can also form silicides – metal-silicon alloys formed with most stable elements including aluminium. Silicon production occurs primarily within abrasives, metallurgical and refractories industries by reacting silica sand with carbon – often powdered coke is used – at temperatures between 2,200-2,700 deg C. The end product of this process can produce mixtures of silicon carbide, carbon monoxide gas as well as volatile silicates.
Biological
Silicon only features in Earth organisms through the formation of silica from oxygen (O2). No living form has ever been known to synthesize bonds between carbon and any other element.
Not only is structural diversity necessary to support biochemistry; chemical functional diversity must also exist. Silicon’s unique chemical properties provide a potential way for such diversity when combined with other elements; for instance, silicon atoms possess larger covalent radius which produces bonds with various lengths and angles, leading to different ring structures and reactions with carbon analogues – properties which contribute significantly to organosilicon molecules like sila-venlafaxine (9).
Oxygen-silicon chemistry proceeds more rapidly in water-based environments due to an excess of molecular oxygen than in carbon dioxide-containing solvents, making oxygen-silicon reactions significantly faster for biochemical reactions involving organic molecules, including proteins and nucleic acids. This fast switching capability makes water an ideal environment for such reactions.
Silicon’s reactivity enables it to form hybrid backbones for protein-like polymers, opening up new opportunities in vivo for this rare heteroatom such as silicon. However, silicon does come with risks: exposure to silica particles can lead to lung diseases called silicosis that primarily afflict construction workers; inhaling silica powder can result in irreversible changes to lung tissue that result in extensive fibrosis and progressive disease; this toxicology poses serious threats both industrial and consumer applications of this technology.
Theoretical
Silicon-based chemistry may seem dull compared to carbon-based chemistry, but that’s simply an illusion. Their reactions are quite similar; both elements produce high molecular weight organic molecules containing carbon as well as other atoms (see Appendix A).
However, due to an excess of oxygen that strongly bonds with silicon molecules in space and time, most silicon chemistry cannot exist safely in water environments, thus limiting its role as an essential heteroatom component of life on earth. This fact limits silicon’s role as an important heteroatom component.
Alpha silica (a-SiC), with its Wurtzite crystal structure, is the predominant form of silicon found in nature; on the other hand, beta silica (b-SiC), with its zinc blende crystal structure, has only limited industrial use.
There are however, pockets of non-polar, reversible chemistry for silicon in aprotic solvents like liquid nitrogen and sulfuric acid. Furthermore, these solvents support the formation of numerous organosilicon polymers with diverse side chains – carboxylic acids which form amphiphilic vesicles or micelles when self-assembled; alkyl groups that dissolve easily in non-polar solvents; etc.
Silicon is known to react explosively with organic carbon scaffolds in aprotic solvents, giving rise to various silicon chemicals called zwitterionic (with both positive and negative charges on its silicon atom) compounds, as well as tricoordinate and pentacoordinate (silicon-bonded to five different atoms simultaneously) compounds.
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