In the game, Chlorine is listed in the “Elements” Category.
In real life, chlorine is a gas at both STP(Standard Temperature and Pressure) and NTP(Normal Temperature and Pressure) aka room temperature, just like Oxygen and Nitrogen. All three gases are elements.
So why are Oxygen and Nitrogen listed in the “Gases” Category but Chlorine is not?
Probably for the same reason that Sodium is in Elements rather than Metals. The categories aren’t strict like that - they map to the typical player’s mental model of the world, not to a chemist’s. Oxygen and Nitrogen are well known gases on earth, but Chlorine isn’t found as a gas, it’s too reactive and so is always part of something else.
Actually, the above is probably being too generous. More likely the categories map to the mental model of whichever individual developer first set up the material tables, and they were haphazardly allocating stuff while trying to meet a deadline for something else. I expect there’s a “TODO: Fix element categories” comment somewhere in the code. 
My own pet peeve is that Ammonia should be in Chemicals rather than Gases. But we could do this all day…
That it’s AMM instead of NH3 bugs me but i survive
NA/CL is a pretty commonly known chemistry code for those items. As is H2O.
NH3 is pretty far down the list in terms of common knowledge. It’s strange because some MAT codes are phonetic, like AMM or BBH… But others are their chemistry code like H2O or AU.
I kind of agree with you though - would be nice to be consistent. But it’s pretty much set in stone as any changes would break… a lot of things. Perhaps on a universe reset, maybe. But there are more important things to spend dev time on.
One might say you’re salty about it.
presumably anyone who can appreciate a bad chemistry joke 
Speaking of wrong categories: Air Scrubber and Water Reclaimer are both produced in APF, contain the same software and electronics, both used to produce Life Support Systems, bur Air Scrubber is Construction Parts and Water Reclaimer is Electronic Systems.