So correct me if I'm wrong, but when searching for extra terrestrial life, scientists usually search for signs they would expect for life on earth to thrive in. We look for conditions on planets that meet certain criteria e.g close enough to a sun for water, environments for carbon based life forms to exist. I was wondering; why cant an alien be a sulphur based life form that for the sake of argument drinks lead and breathes in chlorine like we do oxygen.
I want to start with a disclaimer that I'm not a chemist and hopefully I don't screw any of the chemistry up. If I do, please let me know.
I think Bill Oertell hit the nail on the head. Just because you can imagine a species which can drink lead doesn't mean it's actually physically possible. Only specific atoms can form the basis of the (extremely) complex organic molecules needed to form life and Carbon happens to be the atom which can do that. The primary reason for this is that Carbon has four valence electrons and its outer shell can hold eight, allowing it to form strong and varied bonds that most other atoms just can't form. As such, Carbon is the most logical element to form the backbone of organic molecules.
Other ___-based Lifeforms
Moving down the periodic table, you can see that Silicon has the same properties (4 valence elections, 8 possible) and you might be inclined to think that you could form some sort of silicon based life. This topic has been widely considered, but you run into a number of problems that make Silicon-based life unfeasible. The primary reason is that Silicon is just much bigger and more massive than Carbon (having a nucleus with 28 nucleons compared to Carbon's 12). This makes it harder for Silicon to form the necessary bonds that Carbon can readily form and you're out of luck right form the get-go. Similar issues occur with other "Carbon group" elements, except they're even further exacerbated by even higher masses.
There are many other "organic" configurations chemists have considered such as boron-based life or sulfur-based life but they all have some sort of problem that makes it hard for it to exist. Some are more possible than others, but Carbon-based life, as we know it, just seems to be the best solution. If you gave someone all the elements in the periodic table and said, make organic life, they'd probably come back with Carbon-based life because its the easiest. You don't have to overcome as many obstacles.
Aside from the impracticality of non-Carbon based life, you have to consider that everything we've seen in the universe, as far as organic chemistry goes, points to the fact that the Universe is filled with Carbon-based chemistry. Astronomers have been able to look out at comets, interstellar dust clouds, and all sorts of places and time after time, they see complex, Carbon-based, organic molecules. For example:
- We've found Polycyclic Aromatic Hydrocarbons (PAHs) all over the solar system and galaxy,
- Astronomers have also found things like i-propyl cyanide in meteorites and in interstellar dust $-$ an organic molecule which is a necessary ingredient in amino acids,
- Organic molecules have been found (and believed to have formed) on Ceres.
The point I'm driving at here is that the universe seems ready-made to start forming Carbon-based life everywhere. Everywhere we look, we see organic molecules which have Carbon as the backbone of their structure. Nowhere do we see complex silicon based molecules (or something even crazier). This is highly suggestive that Carbon is the way to go if you want to quickly and easily form life.
There are probably a 100 other reasons to favor Carbon over something else, but that delves into more complicated chemistry than I've touched on here and I'm not as knowledgeable about that. I would think this question would get a more in-depth chemistry answer on the chemistry stack exchange.
Carbon is like lego-technic. It connects water into organic compounds which have every chemical and physical property from volatile solvent to syrup to oil to tar and rock.
Silicon is like lego, try building a car out of it, it's difficult to find a good solvent for it. it can't even bond H. Silane which is the equivalent of methane doesn't occur naturally on our planet, it's too unstable. since silicon doesn't form OH compounds, it doesn't have the malleability of water, it's always rocky and unstable except at -200 degrees where it could be more versatile than carbon. silicon compounds under -180 degrees C are interesting and worth studying, they would need a kryogenic space suit to keep them cool, and if you shoot them they would spray liquid oxygen everywhere.
Sulphur is even less versatile than silicon. it only forms linear chains and is hyper reactive with hydrogen.
You'd need a solvent other than water which is used for carbon based. Ammonia, formaldehyde, sulphuric acid, liquid nitrogen, hydrogen fluoride, methane, hydrocarbons, supercritical fluids.
Perhaps complex robotic life forms are some of the easiest to make up. Life forms that can be designed by artificial intelligence, to use most commonly available resources at ambient temperatures are more likely than complex life forms using other than carbon. That's because a large computer can design transistors/mechanics that work with any metals and conductors in very cold temperatures (aluminium can structurally handle -130 kelvin easily), and robotic life forms would prefer to be in a stable environment without biology, clouds, rain, water, in cold environments, it would be easier for them.
Carbon can hold one of the most stable 4 bond connections out of the elements. We see how life is shaped on earth and based on the properties of life here, we make educated guesses on what life must be based around in other parts of the universe, given that there is other life.
To answer the second part of the second, the reason we breathe in oxygen is to allow certain cellular functions to take place, the most important reason is respiration. C6H12O6 + O2 yields CO2 + H2O + 36-38 ATP. ATP is the molecule the body uses as energy. Oxygen is essential for us because it's used in the chemical reaction that gives our body energy. If an alien were to be breathing chlorine, it would need to have a chemical process that uses the chlorine in their body. As for there being sulphur-based life, I don't know whether sulfur is stable enough to hold the same bonds as carbon. From what scientists know, carbon is the best universal organic base, because of its stable 4 bonds.
An oxygen-based life form may also be possible, given that oxygen readily bonds with other molecules. In addition, how do scientists actually know that the laws of physics, chemistry, and life apply everywhere? Why not just Earth or our solar system? Why not just the Milky Way? There could be different laws and possibilities in different galaxies, which humans will most likely never discover. Who are we to decide what id true and what is not when it will never actually be known? For all I care, life on Earth could be the most simple and unimpressive in the entire universe. We could have the intelligence and lifespan of a maggot in comparison to some form of existence. Life is, after all, just a machine. What should we care anyway? We have to; that's the only known reason as to why life exists. To fill the void. To be there. What's beyond existence? Maybe that's where a black hole leads. In all honesty, this world is far beyond any conceivable understanding, far beyond the reach of any of us. At the end of the day, we are all bodies in the inconceivably large world of being. A fleck of dust in two-dimensional space. In reality, we are only worth something to ourselves.