My science teacher says that all ions are stable, which seems wrong to me.

For example $\ce{He^{-1}}$ is an unstable ion of helium, and although it might not exist in the real world, it is still technically an ion of helium. Is it right?

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    $\begingroup$ Of course there are plenty of unstable ions. But maybe your teacher wanted to say something different. He might have thought of a given group of ions, for a given application... $\endgroup$ – Maurice Apr 29 at 15:53
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    $\begingroup$ Even calling a positively charged ion 'stable' ignores the fact that it is quite happy to grab spare electrons... $\endgroup$ – Jon Custer Apr 29 at 15:59
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    $\begingroup$ +1 for not just swallowing a sweeping statement without thinking through its limitations. // $\ce{He+}$ isn't "stable" either. // The statement would be true for ionic salts. So in NaCl, the sodium and chlorine atoms don't flip-flop between ions and uncharged atoms. $\endgroup$ – MaxW Apr 29 at 16:03
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    $\begingroup$ In a way, everything that exists is stable. $\endgroup$ – Ivan Neretin Apr 29 at 16:24
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    $\begingroup$ I will point out that the fact that the negative ion of helium is metastable for long enough is important for tandem accelerators - one can do Rutherford Backscattering with helium since the metastable lifetime is long enough to get to the terminal to then be stripped to being a positive ion. So, indeed, the negative ion of helium 'exists' at various times around the world. $\endgroup$ – Jon Custer Apr 29 at 17:01

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