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As a total chemistry layman I enjoyed reading "Why doesn't $\ce{H2O}$ burn?", but it prompted another question in my mind. One of the answers there was that $\ce{H2O}$ can burn in the presence of a stronger oxidizer like fluorine, so burning stable compounds is just a question of using a stronger oxidizers.

Or is it? Is there a chemical species, or a "least reactive compound" that, once made, is difficult or impossible to chemically transform into something else? A chemical black hole, if you will.

Does the above answer change if we allow high temperatures and pressures versus restricting ourselves to roughly standard temperature and pressure?

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    $\begingroup$ No. You need to specify reactivity towards what. $\endgroup$ – bon Dec 18 '16 at 9:03
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    $\begingroup$ There is neither least reactive compound, nor the most reactive compound, and indeed the very word "reactive" is not well-defined to the point that you'd better not use it at all. $\endgroup$ – Ivan Neretin Dec 18 '16 at 9:29
  • $\begingroup$ Perhaps the question should be rephrased to use "most inert" rather than "least reactive." There is no single compound that is always the most inert, but there are definitely examples of compounds that are unusually inert with regard to other chemicals. Fluorocarbons like teflon and CFCs are good examples of "inert" compounds. $\endgroup$ – barbecue Dec 18 '16 at 17:56
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    $\begingroup$ @IvanNeretin Could you suggest a better term? I haven't had any chemistry since high school (and about 12 years ago). $\endgroup$ – David Dec 19 '16 at 4:18
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    $\begingroup$ @david a lot of fluorocarbons are highly resistant to being broken down by chemicals alone. That's why PTFE is often used for storage containers for strong acids and bases, for example. And yes, changing temperature and pressure definitely changes the whole equation. They can easily be broken down by high heat. $\endgroup$ – barbecue Dec 19 '16 at 22:14
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I think a good argument can be made for either helium or neon, the most noble of the noble gasses. Those are the two prototypical unreactive elements. They are the only two stable elements for which no more complex compounds (i.e., other than the single atoms themselves) have yet been isolated, at any temperature. The slightly more reactive element argon will admit formation of compounds such as argon hydrofluoride ($\ce{HArF}$). This compound is only stable up to $\mathrm{17\ K}$, because any hotter and the frail bonds are overcome by random thermal collisions which break the compound apart into $\ce{Ar}$ and $\ce{HF}$.

Picking which of helium or neon is less reactive is a bit more difficult. A naive analysis of periodic trends would point to helium as the most inert, but more detailed computational studies suggest that at least in some cases neon may be less reactive. For example, the extremely Lewis acidic compound beryllium monoxide ($\ce{BeO}$) may potentially form an isolable, if very weakly bound, compound with helium, $\ce{HeBeO}$, but neon is not thought to form the analogous $\ce{NeBeO}$. $\ce{HHeF}$ may also be just barely stable, whereas $\ce{HNeF}$ is not thought to form. None of these have yet been observed in the laboratory, but there is definitely ongoing research into coaxing helium and neon to make isolable compounds.

All that said, we can get helium and neon to react, if we drop the requirement that the product must be isolated (that is, "put into a bottle"). Chemical species such as $\ce{He2^+}$, $\ce{Ne2^+}$ and $\ce{HeNe^+}$ have long been known from mass spectrometry experiments, they just can't be isolated because that would require the presence of a counterbalancing negative ion, which would immediately proceed to react with the positive ion and cause decomposition with release of the noble gas. You can also bring what is arguably the most reactive species in chemistry, the hydrogen cation ($\ce{H^+}$) into the fray. Helium and neon will both easily react with $\ce{H^+}$ to form $\ce{HeH+}$ and $\ce{NeH+}$ , as shown by the exothermic proton affinities of $\ce{He}$ and $\ce{Ne}$. Again, these composite particles cannot be isolated, as they are amongst the strongest Brønsted-Lowry acids in existence, and will protonate anything they come into contact with in order to release the neutral noble gas atom. Some other possible non-isolable relevant noble gas ions are $\ce{FHeO^{-}}$, $\ce{HeCCH+}$ and $\ce{PbHe15^{2+}}$ (!), among others.

Edit: I forgot to mention that there are a few other cases of helium or neon binding to other atoms. By exciting one of the electrons in the atom, it is possible to coax it to bond with another one, forming an excimer or exciplex. See for example, the dihelium excimer $\mathrm{He_2^{*}}$. This is a short-lived species that still can't be isolated, however, because in a matter of microseconds it releases a photon and de-excites, promptly separating into the two free atoms.

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    $\begingroup$ "isolable compounds" ? Not really. Matrix isolation is ... fine, but not really "put in a bottle", right? ;-) $\endgroup$ – Karl Dec 18 '16 at 12:46
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    $\begingroup$ The question specified compounds, which implies two or more elements together, rather than molecules of a single element. $\endgroup$ – barbecue Dec 18 '16 at 17:52
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    $\begingroup$ Good answer. I suppose the intent behind my original question was whether or not it was possible to create a molecule so inert that it wasn't really possible to recover the constituent components without extraordinary measures. This is probably why using the term "reactivity" was a bad idea. $\endgroup$ – David Dec 19 '16 at 4:41
  • $\begingroup$ @Karl The bottle is part of the Matrix! ;) $\endgroup$ – Jan Dec 20 '16 at 1:29
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If you want to limit the answer to "compounds", I would suggest sulfur hexafluoride and nitrogen gas as two of the most unreactive small molecules. Some high temperature ceramics might also qualify, although they are not really small molecules. Interesting question! - Which begs another question: how do you quantify how unreactive a compound is?

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