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Primordial Earth was buried in carbon dioxide. When life started, it was with algae using energy from the sun to crack the carbon out of carbon dioxide, with the byproduct of releasing the corrosive and toxic oxygen gas into the air. From there we get animals that use said oxygen, and so on with increasing complexity.

So, to get sapient species that breathe fluorine gas in a similar manner as we do oxygen, we must begin at the beginning, with plants. What compounds would need to exist in abundance on a primordial planet in order for a photosynthetic reaction that results in fluorine as a waste product to occur?

Gio
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Fluorine gas burns the ashes that other reactions leave behind

Given the absurd reactivity of fluorine, anything it comes in contact with will ignite. Attempting to develop a system of biochemistry, which requires long chains of stable links, on an element that burns everything it touches is an exercise in futility.

While it's true that oxygen is very reactive, it's unreactive enough at the temperatures that life currently lives that it's a useful oxidizer. Fluorine is too reactive. Fluorine gas reacts with carbon at room temperatures. Fluorine reacts with hydrogen very energetically (assumed at room temperature). Hydrogen plus fluorine plus water is hydrofluoric acid, an especially nasty chemical. At elevated temperatures, fluorine will react with noble gases and metals which ordinarily don't react with anything!

On Earth, fluorine is only found in minerals. What did appear in the atmosphere reacted with water or carbon di/mon-oxide almost immediately.

While there are some fluoride metals, these metals don't appear to offer the complexity that carbon-oxygen-nitrogen-hydrogen chemistry does. As such, I'm highly skeptical that you could base biochemistry in an atmosphere with any substantial amount of fluorine.

Fun Reading

Green
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    This isn't an answer to the question. – sphennings May 09 '17 at 19:54
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    Assuming this is an attempt to challenge the premise, it seems like it might be valid. Having an extremely reactive byproduct to any chemical reaction without any good way to dispose of it is not good for life continuing to use that reaction. – HDE 226868 May 09 '17 at 19:56
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    If it is challenging the premise it should follow through and explain why flourine wouldn't be the byproduct of a photosynthetic reaction. – sphennings May 09 '17 at 20:02
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    @sphennings, challenge accepted :) – Green May 09 '17 at 20:03
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    @sphennings but it does! Fluorine burns everything. Even water. It can't be byproduct, unless organism is doing it once and dies, but that's kinda bad. And you can't have it free in atmosphere unless everything on the surface is fluorine compound - because anything exposed would get burnt. – Mołot May 09 '17 at 20:06
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    @Mołot That is an excellent explanation. – sphennings May 09 '17 at 20:12
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    @Green Thanks for completing your answer. :) – sphennings May 09 '17 at 20:13
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    If elevated temperatures causes fluorine to react with noble gases, what happens at decreased temperatures? Could fluorine biochemistry be more realistic on a frozen word? – Rob Watts May 09 '17 at 20:22
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    @RobWatts, fluorine might calm down near absolute zero but at that point, no other reactions are going to happen either. Given that its melting point is 58Kelvin, not much is gonna happen. – Green May 09 '17 at 20:25
  • Thanks Green! I marked this as an answer and added the "reality-check" tag. – Gio May 09 '17 at 21:02
  • @sphennings Heck the stuff even oxidizes Oxygen. Although Oxygen Fluorides are evil chemicals that evoke images of Brimstone and Pitchforks, especially FOOF. – Aron May 10 '17 at 01:00
  • @Mołot, what kind of a hellscape is that where everything is a fluorine compound? – Green May 10 '17 at 01:09
  • The "just reactive enough" qualities of carbon and oxygen are part of the reason why any extraterrestrial life we find will almost certainly be based on them. – RonJohn May 10 '17 at 01:35
  • What does the periodic table look and act like in a world where antimatter is the norm and matter is exotic? Could, in that world, fluorine, if it exists, react with other elements differently? Just trying to imagine a world where this concept could be feasible, since it was asked on Worldbuilding. Although the realty-check tag without an alternate dimension tag makes that a bit more difficult.... – N2ition May 10 '17 at 04:49
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    @N2ition no. Antimatter is chemically the same. – JDługosz May 10 '17 at 10:59
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    @N2ition chemically, antimatter behaves exactly the same as matter. The difference is that in the presence of matter, antimatter makes a very big boom. – Green May 10 '17 at 11:07
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So..

How does our planet look like, first?

For some reason - and I really can't think of a good one, perhaps some extreme fractionation in a nebula - we have no oxygen around. Oxygen messes things up in this case. Instead we have Fluorine, which is always more fun.

So instead of rocks, we have Silicon fluoride rocks (Si(n)F(2n)F2). Instead of oceans of water, we have oceans of HF. And instead of an oxygen atmosphere, we have a Fluorine atmosphere. The surface temperature would be something like -100 degrees centigrade - this does help by lowering reaction rates.

Organic chemistry is hard in this environment. It would have to be based around Carbon-Fluorine polymers, which would be stable, as opposed to our Carbon-Hydrogen-Oxygen polymers, which would react violently. Presumably some trace oxygen and nitrogen would have to be available to allow more diverse chemistry; the interior of cells would have to do a lot of work to avoid the HF destroying everything.

The equivalent of CO2 in this world would be CF4 gas, which would be reacted through photosynthesis into C(n)F(2n) polymers with F2 being released. This is one of the more straightforward parts, although you'd want very high energy wavelengths of light to drive it, so perhaps our plants would look red rather than green.

One thing I would not want to do is try and land on this planet..

Andrew Dodds
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