The Chemical Tale of Silicon

Atoms 

(skip this if you still remember school chemistry)

figure 1 - natrium

All atoms contain negative particles called electrons, which reside in different tiers or energy levels inside the atom. These energy levels (which are also called shells) are nested concentrically inside each other [figure 1], and each level needs a certain number of electrons to fill itself and feel satisfied. In the innermost level closest to the nucleus (contains protons and neutrons, which make the atom mass), the number is two. In higher levels, it's usually eight. Atoms fill their inner (and lower) energy levels as full as possible with their own electrons, and shed/ steal electrons to secure the satisfying number on the outermost level. This outermost energy level / shell is called the valence shell, of which its electrons are most likely to account for the nature of any reactions involving the atom and of the bonding interactions it has with other atoms.

figure 2 - helium

Helium, element two, has exactly the number of electrons it needs to fill its only level [figure 2], making it an element with a closed shell configuration. Closed shell of valence electrons tends to be chemically inert; all the elements with closed shells (full complements of electrons) will not react with anything under normal conditions. An atom with one or two valence electrons more  is highly reactive, because the extra valence electrons are easily removed to form a positive ion. An atom with one or two valence electrons fewer than a closed shell is also highly reactive, because of a tendency either to gain the missing valence electrons (thereby forming a negative ion), or to share valence electrons (thereby forming a covalent bond).

Note that these electrons inside a shell are arranged in orbitals (s, p, d, f orbitals; of which the letters originally were used to classify spectra descriptively into series called sharp, principal, diffuse, and fundamental). You can learn more about it here:

• https://www.britannica.com/science/orbital
• shells, subshells, and orbitals

Carbon and Silicon Brotherhood

Let's take a look at the famous periodic table of elements [figure 3] and find carbon. In periodic table, the vertical columns are called groups or families because of their similar chemical behavior (same number of valence electrons and similar chemical properties). Thus carbon has more in common with the element below it, silicon, than with its two horizontal neighbors: boron and nitrogen.

figure 4 - amino acid structure

Carbon is known as the most versatile element (has the capability of combining with many other elements -- more on this later) and can be found in many compounds (like amino acids for example).

Each amino acid contains oxygen on one end, a nitrogen on the other, and a trunk of two carbon atoms in the middle (it also contain hydrogen and a branch off the main trunk that can be twenty different molecules) [figure 4]. Comparing the elements in amino acid, oxygen is element 8 on the periodic table meaning it has 8 total electrons. 2 are inside the innermost shell (the lowest energy tier) which fills first. The outermost shell has 6, so oxygen wants 2 additional electrons [figure 5]. Two electrons aren't that hard to find, but that's not the case for carbon.

Carbon, being element 6 fills in its inner shell with 2 electrons leaving 4 valence electrons, therefore it needs 4 more to make 8. It is harder to do and the upshot is that carbon has really low standards for forming bonds: it latches onto virtually anything and is promiscuous, but it turns out that the promiscuity of carbon is actually a virtue: carbon can bond with other atoms in more directions than oxygen (it can share its electrons with up to 4 other atoms at once), hence allowing carbon to build more complex chains and even 3D molecules. Carbon shares and cannot steal electrons making the bonds it forms steady and stable.

Silicon as element 14 (2-8-4) also has 4 valence electrons (the same predicament as carbon giving silicon some of carbon's flexibility too). Carbon's flexibility is directly linked to its capacity to form life and silicon's capability to mimic carbon has made it appears in science fiction as carbon's alternative. But can silicon do the wondrous tricks carbon can?

Just like carbon based life forms, silicon life forms would need to shuttle silicon into and out of their bodies to repair tissues etc. On earth, creatures at the base of the food chain can do that using gaseous carbon dioxide. Silicon always bonds with oxygen in nature, too. BUT it is in solid form silicon dioxide (also called silica), not a gas, at any temperature friendly to life (it only becomes gas at ~2200 °C). On the cellular respiration level, breathing solid just doesn't work because solids stick together and it's hard for cells to get individual molecules from them as what they need to do. Couldn't those silicon creatures expel/suck up silica in other ways? Maybe, but silica doesn't dissolve in water; the most abundant liquid in the universe by far. So those creatures should not have the evolutionary advantages of blood or any other liquid to circulate nutrients and waste.

Furthermore, silicon has 8 more electrons than carbon making it bulkier. Carbon can contorts itself into ringed molecules we call sugars, but rings are states of high tension (they store lots of energy), and silicon isn't supple enough to bend into the right position to form rings. Silicon also cannot squeeze their electrons into tight spaces to form double bonds which appear in virtually every complicated biochemical meaning that silicon based life would have hundreds of fewer options for storing chemical energy and making chemical hormones. Turns out that carbon and silicon are not twins but brothers with differences making silicon harder (maybe not impossible) to form life like its brother.

Silicon Valley or Germanium Valley

Silicon Valley was named for the silicon needed to make semiconductor computer chips. Looking back to the periodic table, right under silicon there is Germanium and if things had gone differently in the past, we might have Germanium Valley in northern California today. The modern semiconductor industry began in 1945 in Bell Labs, New Jersey. An electrical engineer and physicist named William Shockley was trying to build a small silicon amplifier to replace vacuum tubes in mainframe computers because vacuum tubes were cumbersome, fragile, and prone to overheating. Despite everything, vacuum tubes play an important double duty to amplify electronic signals and as a one-way gates for electricity. Shockley with his silicon amplifier never amplified anything and being frustrated after 2 years, he dumped the task to his two underlings, John Bardeen and Walter Brattain. 

Bardeen and Brattain soon determined that silicon was too brittle and difficult to purify to work as an amp. They then tried using germanium to build the first solid-state (as opposed to vacuum) amplifier in 1947 and call it transistor by knowing that germanium,  whose outer electrons sit in a higher energy level than silicon (because it fills more tier), are more loosely bound (more distance between the valence electrons and nucleus, or a greater atomic radius); causing it to conduct electricity more smoothly.

By 1954, the transistor industry had mushroomed. Throughout the boom, engineers kept ogling silicon because germanium was temperamental (it stalls at high temperature) and because silicon (the main component of sand) is cheaper until one day at a semiconductor trade meeting Gordon Teal demonstrated that silicon transistor is possible. That moment, germanium had beed dumped. In 2000, Jack Kilby won a Nobel Prize for his integrated circuit, which was made using germanium since Kilby didn't trust the purity of silicon to make resistors and capacitors. Sadly, though, nothing could resurrect germanium's reputation because silicon was too cheap and too available. So after germanium did all the work, silicon become an icon with geranium banished to periodic table of obscurity.

Silicon in Plastic Surgery?

There's misunderstanding about silicon being used in plastic surgery. It is actually not pure silicon but silicones(with e). Silicon is an element and silicones are polymers containing silicon and oxygen, and often carbon and hydrogen as well. These silicones, also known as polysiloxanes, are a family of man-made polymers that are usually liquid or a flexible, rubberlike plastic. Silicones are used in sealants; adhesives; lubricants; medical products; both cosmetic and orthopedic implants; cooking utensils; tools; thermal and electrical insulation; coatings for paper, textiles, and gaskets; potting for electronics; and even as a dry-cleaning solvent.