Chlorine

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Chlorine is pretty abundant and also accessible (sea water).

Chlorine + carbon = bad

Chlorine is very common inside the human body in the form of free solvated ions (salt). Interestingly albeit chlorine can form very strong bonds to carbon these bonds are not used by biological systems (akin to phosphorus). Conversely when carbon is burnt in conjunction with chlorine these direct covalent carbon chlorine bonds can form and create chlorinated organic compoounds which are highly toxic (those are among others the infamous dioxines [1]). This is why PVC plastic is mostly banned.

Atomically precise technology products can trap chlorine

Similarly in advanced atomically precise technology combining high amounts of both carbon and chlorine might be a bad idea (small traces should be ok). But if enough silicon or metals are present in the product to make the product incombustible high concentrations of chlorine might be less of a problem. Sodium could be included such that even if to near evaporation temperatures the chlorine does not outgass. Note that incombustible products may be harder to recycle.

Details

Chlorine is of special value since in benign chemical environments it features exactly one covalent bond (just like hydrogen and fluorine) and can thus be used to plug open dangling bonds closed. Chlorine is a lot bigger than hydrogen so the shape of the surface passivation layer can be adjusted replacing hydrogen.

Chlorine vs Fluorine -- toxicity, abundance, volatility, ...

Unlike the second period elements carbon, nitrogen and oxygen where compounds with the chemical analogous elements of the third period (silicon, phosphorus, sulfur) are usually more toxic, the situation with chlorine (third period) and fluorine (second period) is reversed.

Carbon and nitrogen, despite being not too abundant in earths crust, are volatile elements that tend to accumulate in the atmosphere from where they where well acessible for life. Life not only got used to these two elements, it uses them as the core building material. Fluorine (clorines lighter analogon) on the other hand is so electronegative and reactive that its a very nonvolatile element usually strongly bond in rocks. In combination with its limited abundance it was and is mostly kept in the lithosphere away from the biosphere. Sonlife nevervget used to dealing with it in high quantities.

Oxygen, as pretty reactive intermediate between nitrogen and fluorine, is a special case. Due to its massive abundance and strong metastabiliy in its volatile diatomic form life eventually had to learn to deal with it.

In contrast to fluorine chlorine (as a third period element) is less electronegative and forms weaker bonds. Making it more volatile (especially in the sense of solubility in water) and thus better accessible to life.

Both Chlorine and Fluorine form metastable diatomic gasses, with fluorine less metastable and more reactive. Both more reaxtive than diatomic oxygen.