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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1$\begingroup$ $\ce{H2SO4}$ is electrically neutral. When two protons are extracted from it, the electron pair that belonged to the two $\ce{H-O}$ bonds are donated to the oxygen atoms, resulting in a negative charge for both of them. $\endgroup$– Sam202Dec 3, 2022 at 4:50
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1$\begingroup$ This is a wrong question to ask, in that the literal answer to it is useless. What causes things to suddenly gain electrons? Electrons, that's what. You see electrons, you grab them. There ain't no such thing as a free electron. $\endgroup$– Ivan NeretinDec 3, 2022 at 16:59
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$\begingroup$ @IvanNeretin I answered the question in the title, which is not a wrong question. It is very broad, however, so I kept my answer specific to the example mentioned. $\endgroup$– Karsten ♦Dec 3, 2022 at 18:26
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2$\begingroup$ @Karsten Your answer is fine. The body of the question, though, still raises my objection. $\endgroup$– Ivan NeretinDec 3, 2022 at 18:54
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But what causes it to suddenly gain electrons and become an ion?
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.
[...] such as $\ce{SO4^2-}$
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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1$\begingroup$ Groan, you shouldn't answer something like that without clarification. OP probably just doesn't know what dissociation is. There are good reasons why we close questions like this one. $\endgroup$– MithoronDec 5, 2022 at 1:09