>>11136984Because Cl- is essentially Ar, but with a negative charge instead of neutral.
First means that, unlike Cl2 but very much like Ar, it has little interest in oxidizing or radicalizing the shit out of you, which is what makes Cl2 so dangerous.
Second lets it actually be useful, unlike Ar. See, this tiny ball of being fully content with its electrons is pretty good at attracting other, positively-charged, balls of being fully content with their electrons — that is, metals that got rid of extra electrons. So if you have some water with Na+ in it, and dry it up, you get fucking nothing because electrostatics means you can't have a non-negligible amount of same-charge ions without them scattering each other (unless you fuck with plasma and magnetic traps and stuff, which is not chemistry); but if you have some water with Na+ and Cl- and dry it up, Na+ conveniently get attracted to Cl- and form a clump that's all + on the outside and thus attracts more Cl- which attract more Na+, and all of this settles into the lowest-energy possible configuration, which in this case is dense, regular, crystallic and good with boiled eggs.
More importantly to the "essential nutrient" part, if you're a cell and you want to push some electric charge to one side of some membrane (say, if you want to use resulting difference in charge to bend that membrane's shape, or even to discharge it into some kind of signal), you can't actually push electrons through it, since "an electron" is not something you can interact chemically with and radicalized organics are dangerous. Cl- is big, carries the same 1e- worth of charge and isn't going to part with it, so you can push it through your membranes all day long. Lots of other ions could do the same, but Cl- is highly solvable unlike SiO3 -2, safer to handle than F-, less likely to give up the electrons and become something else than S-2, and unlike heavier halogens used to be as avaiable as it gets back when we dwelled in seawater.
