If you watched Finding Life Beyond Earth last week, you might be wondering, what's the big deal about carbon-based life? Spock discovered silicon-based life in episode twenty-five of the original Star Trek series. And Star Wars' bounty hunter Zuckuss is ammonium based. How can scientists say these two hegemons of geek culture are wrong? Why does life have to be based on organic molecules?
The answer to this question is actually right in front of you. I'll give you a hint--it's big, colorful, and located directly below this paragraph. It's the periodic table! The information in the periodic table of elements is sufficient to convince scientists that life has to be carbon based--you just have to know how to interpret it.
The answer is in here! Image via Wikipedia
Let's start with carbon's box on the periodic table. Carbon is element number six, which tells us that it has six negatively charged electrons buzzing around six positively charged protons in the nucleus. That's the information we're given--now let's apply a basic knowledge of chemistry and interpret it.
Atoms house their electrons in concentric energy shells. The first energy shell holds two electrons, the second and third hold eight. (There are more, but let's stop here for now.) Atoms fill their shells sequentially, and they always like to end with a full shell--even if that means giving away electrons. Sodium, for instance, has one electron dangling in its third energy shell and instead of trying to gain seven more electrons, sodium simply gives this electron away. Other atoms steal electrons to fill their outermost shell. Chlorine is one electron short of filling its outer shell, so it plucks an electron from an element, like sodium, and turns into a stable, negatively-charged version of itself.
But poor carbon has four electrons in its outermost shell. Losing all four electrons is unthinkable, but gaining four is nearly impossible. The only way carbon can fill this shell is to share its electrons with other atoms. Every time carbon picks up a new electron, it doesn't just get the electron, but the entire atom! This process secures carbon to other atoms in what is known as a covalent bond. Carbon can share up to three electrons with a single atom, making it capable of forming single, double, and triple bonds.
Carbon is so desperate for those four electrons that it has low standards for bonding. Some atoms, like oxygen, are pickier and will only bond if the right element comes along. But carbon can (and will) bond to almost anything with unpaired electrons. The upside of this atomic promiscuity is that carbon can form varied and complex structures. Carbon atoms can link together into long chains, rings, and complex three-dimensional structures, or bond with other atoms like nitrogen and oxygen to create varied molecules with a diversity of functions.
But if you're looking at the periodic table, you might be wondering, "Silicon has four electrons in its outermost shell. How come life isn't silicon based?" Good point! Many people (including the writers of Star Trek) have speculated that life could be based on silicon. But we don't see rock creatures scooting around earth because silicon is a less aggressive bonder than carbon. Silicon has an atomic number of fourteen--fourteen electrons and fourteen protons. Instead of wanting to fill its second shell (like carbon,) silicon needs to fill its third shell. The third electron shell is much further away from the nucleus and therefore its electrons feel a weaker pull towards the nucleus.
To visualize this, imagine a group of students (electrons) surrounding a pizza (nucleus), with one slice of pizza for each student. When the pizza is small, the group of students is small and they can huddle together. But if the pizza is big, the group of students is big, and some students inevitably get pushed to the outside ring. In this outside ring they can't see or smell the pizza, and thus they tend to wander. If silicon's own electrons are wandering, why would electrons from other atoms want to join them? It comes down to this--silicon is just not as "attractive" as carbon (yes, pun intended). Silicon still bonds, but the bonds are weaker and more monotonous. Sure, silicon makes great rocks, but not great building blocks!
At this point, you're probably thinking, "But there are ninety more elements on the periodic table. Surely one of them must have chemistry capable of life?" This brings us to the fun part of the article--the elemental elimination game!
Let's start with everything left of the column containing carbon: the alkali, transition, and semi-metals. To begin with, most of these elements have weak holds on their outer most electrons and can exist in positively-charged forms. If life were made out of these types of atoms, we could potentially dissolve in water, which clearly wouldn't work. (Just ask the Wicked Witch of the West.) Besides, most of these atoms prefer to exist in crystalline structures.
This alone eliminates 80% of the naturally occurring elements. Now let's move to the far right side of the periodic table. Take a look at fluorine: all of the elements in and to the right of this column are either already full with electrons or so greedy that they steal electrons with no intention of sharing. Fluorine and chlorine will occasionally slip into "electron sharing" relationships, but they are much more fickle than steadfast carbon, and life cannot be built on a capricious element.
Also, in the atomic world, size does matter, so we can eliminate all the elements that are too big to do the job right. Elements like arsenic and selenium are like silicon--bulky, clumsy, and just not as attractive as carbon. When these elements slip into organic molecules, they often cause more trouble than good. (Arsenic is highly toxic because it can, in fact, replace phosphorus in organic reactions, but its large size hinders organic processes.) So now we're left with five elements: carbon, nitrogen, oxygen, phosphorus, and sulfur, and guess what? Each one of these elements is highly abundant in organic material. But out of these five, carbon is the only one capable of holding four stable bonds and linking into long chains with other carbon atoms. These last four elements are perfectly suited as organic sidekicks, but they lack the versatility, fortitude, and ubiquitous nature that make carbon the backbone of life.