How are polyatomic ions formed? [closed]








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I know that there should be a covalent bond between polyatomic ions, such as $\ce{SO4^2-}$. But what causes it to suddenly gain electrons and become an ion?





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> But what causes it to suddenly gain electrons and become an ion?
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In principle, you can get a polyatomic ion by adding atoms to a monoatomic ion, or by changing the number of electrons in a (neutral) molecule, or by splitting a (neutral) molecule into positive and negative ions, or by reactions of other ions.



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> [...] such as $\ce{SO4^2-}$
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I am not aware of the existence of $\ce{SO4}$, so just adding two electrons to a molecule probably doesn't happen in this case. Here are some formal scenarios to get sulfate, some of them more conceptual than realistic:
$$\ce{HSO4-(aq) <=> H+(aq) + SO4^2-(aq)}$$
$$\ce{S^2-(aq) + 2O2(g) -> SO4^2-(aq)}$$
$$\ce{Na2SO4(s) -> 2Na+(aq) + SO4^2-(aq)}$$
$$\ce{S2O8^2−(aq) +2I-(aq) -> 2SO4^2−(aq) + I2(aq)}$$
$$\ce{SO3(g) + 2OH-(aq) -> SO4^2- + H2O(l)}$$


Some of these reactions are redox (electron transfer), some are acid/base (proton transfer), some are dissociation or hydration, and some are combinations of these.


Here is a reference for the second scenario (which looked unlikely to me so I looked it up), with a lot of different sulfur-containing ions as intermediates: <https://doi.org/10.5935/0103-5053.20160197>


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