I want to create a character or a race that cannot breathe oxygen or carbon dioxide. What other gases could this person or race breathe? The gas doesn't have to be in the air like oxygen is. I'd like to have these characters wear full helmets or gas masks with a tube that leads to a gas tank they carry on their backs.
You could google atmospheric composition, if you are talking about gases in the Earth's atmosphere. Nitrogen and argon are the two other major components of our air. Nitrogen is relatively unreactive, b ut could conceivably have a biological function for an alien species. Argon is an inert gas, and would therefore be biologically pretty useless as well as being unreactive. I have no idea what kind of biology your aliens have. A silicon-based life form would probably be unable to tolerate oxygen, although carbon dioxide would be less likely to be a problem. Your alien would need more than a breathing mask, though. Oxygen would probably be a problem on exposed "skin" as well.
Google H2S Its deadly to us humans. I think that would put some sort of crazy twist on things. Its also a natural gas, its not man made. We have to deal with that gas all the time out here in the oil field. You get a good couple of whiffs of that stuff and you a done deal
Why not a fluid instead of a gas? The container is pressured anyway to a fluid, to reduce volume. The biology might be fish-like then. HTH
Yes, hydrogen sulfide is on the same order of toxicity as hydrogen cyanide. Hydrogen sulfide is the odor of rotten eggs, but it can quickly deaden the olfactory nerves so you stop smelling it. Methane and ammonia are common gases on planets that do not have plants (and therefore little or no free oxygen). Chlorine or fluorine are highly corrosive gases that are toxic to terrestrial life forms. Sulfur dioxide is another gar humans cannot tolerate, but some bacteria can metabolize. There are any number of gases that could exist in an alien ecology, but would not be found naturally on Earth.
I'd like to have a gas that could either be natural on an earth-like planet (because this race would reside beside other races that breathe oxygen), or as Leonardo said, a liquid instead of a gas. I was thinking this race could have surgically inserted tubes into the sides of their necks or faces, with tiny, horizontal slits where a human's mouth would be as a way to speak words.
What about a race that breathes Liquid Mercury? It's naturally occurring but can be deadly to other races and it could be carried in portable "breathing tanks".
Not really. It could, in a reducing atmosphere, but mercury is sufficiently reactive it is only found in nature as compounds on Earth, or any atmosphere of oxidizing gases. The problem is, if you simply pick a compound at random, the chemistry might make no sense whatsoever. You would at least need toi know what kinds of chemicals could coexist with it, in what temperature ranges, and whether it could reasonably expected to take place in a metabolic reaction (reactive enough, but not so reactive that the products are too inert). The chemistry I'm talking about is not necessarily at college/professional level, but certainly at advanced high school level. You can expect a decent proportion of your readers to know that much chemistry, so you will look foolish if your chemistry isn't at least somewhat plausible. On the other hand, I may be expecting too much. The TV series Alien Nation had aliens who reacted to seawater like being dumped in a vat of acid, and yet they could tolerate substantial contact with human blood.
