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1 Robot: Greetings, friend aliens.
1 Iki Piki: A robot!
2 Robot: Please allow me to show you around the ship.
2 Iki Piki: Er.. okay.
3 Robot: This is the recreation lounge. Over there are personal hygiene facilities, fitted out for both oxygen and chlorine breathers.
3 Serron: Convenient...
4 Robot: We have a 600 gigawatt hyperdrive capable of 0.85 parsecs per day.
4 Iki Piki: Wait. Are you trying to sell us this ship?
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One of the popular staples of science fiction is aliens who breathe chlorine. This is chemically somewhat plausible, since chlorine is a gas at a large range of temperatures and pressures, and because it is an oxidising agent - meaning that it reacts with other elements in a way broadly similar to oxygen. Chlorine can combine with hydrocarbon compounds to release energy, in an analogous way to oxygen combining with the hydrocarbons in food.
One problem is that chlorine is simply more corrosive than oxygen. Oxygen is in fact a fairly nasty chemical. It rusts iron, corrodes copper, and causes many other metals to tarnish or otherwise corrode. It combines with many, many common chemical compounds in a violently energetic and dangerous reaction - which we call fire. And there are some compounds it combines with even more violently, causing what we know as explosions. It can produce nasty byproducts in reactions within our bodies, which eat away at the cells of our bodies, causing damage to them, leading to degenerative diseases of the brain, muscles, and nervous system. (This is why antioxidants - things that prevent this effect of oxygen - are a good thing to have in your body.) Oxygen is dangerous, dangerous stuff.
But we live in a sea of oxygen. 21% of the air around us is oxygen. Which means that we've evolved to deal with it, more or less. The oxygen actually got there as a byproduct of the early life on Earth, which had to deal with an atmosphere mostly made of nitrogen, carbon dioxide, and water vapour, with a bit of methane and almost no free oxygen. The first bacteria metabolised by photosynthesis, releasing oxygen until we ended up with the atmosphere we have today.
If life got started on a planet rich in chlorine, it's not inconceivable that such life would use that chlorine as a reactant to metabolise its food, and of course would evolve in a way such that it wouldn't be poisoned or corroded by the gas.
The main problem with chlorine-breathing life is that planets with an abundance of chlorine are also going to have a heck of a lot more oxygen. Throughout the universe, oxygen is the third most common element, after hydrogen and helium. Planets will naturally have lots of oxygen. Chlorine, on the other hand, is approximately 4000 times less abundant than oxygen. It's heavier than oxygen, meaning that if a planet smaller than Earth (such as, say, Mercury) manages to get some chlorine into an atmosphere, it could stay there while any gaseous oxygen drifts off into space. But the problem is there simply isn't enough chlorine in the first place to even make a decent chlorine atmosphere. And life might get started based on chemical reactions that release chlorine rather than oxygen, but that's harder to do chemically (it takes more energy), and less likely than using all the oxygen compounds that will also be lying around for the taking, and in greater abundance.
We have chlorine on Earth. Quite a lot. But we have thousands of times more oxygen.
It's nice to speculate and fantasise. But chlorine planets, and hence chlorine breathers, are very unlikely to exist in reality.
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