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Based on WebElements, of the noble gases, $\ce{He}$ and $\ce{Ne}$ do not react with any of the halogens; however:

  • $\ce{HArF}$ has been detected at low temperatures (thank you to @bon and @Martin in the comments)
  • $\ce{KrF2}$ can be synthesised at low temperatures.
  • Xenon can react with fluorine to produce $\ce{XeF2}$, $\ce{XeF4}$ and $\ce{XeF6}$
  • Radon can react with fluorine to produce $\ce{RnF2}$

I understand that fluorine is the only halogen that seems to react with the noble gases (partly due to the related question Why Aren't Chlorides Of The Noble Gases As Prevalent As Their Fluorides?. My question is

Why are there more fluoride compounds formed with xenon compared to the other noble gases?

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    $\begingroup$ Do you know the three-center four-elcetron bound ? $\endgroup$
    – ParaH2
    Commented May 22, 2015 at 16:12
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    $\begingroup$ Also $\ce{HArF}$ has been detected at very low temperatures and confirmed to have proper bonds between the atoms. $\endgroup$
    – bon
    Commented May 22, 2015 at 17:24
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    $\begingroup$ Calculations for HArF show that there is a covalent HAr and an ionic ArF interaction: nature.com/nature/journal/v406/n6798/full/406836a0.html $\endgroup$
    – Martin - マーチン ♦
    Commented Jun 30, 2015 at 13:38
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    $\begingroup$ Thank you @bon and Martin, I have added this compound into the question $\endgroup$
    – user15489
    Commented Jun 30, 2015 at 13:55

2 Answers 2

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The reason that we discovered more fluorides of xeon than helium, argon, and krypton is quite obvious and you might already know it. As lighter noble gases have more stable shell configuration, it is more difficult to make any kind of compound, including fluorides.

Also, the higher amount of fluorine atoms in a molecule of the compounds means drawing more electron from the noble gas, which is thermodynamically not preferred. Thus, $\ce{KrF4}$ and $\ce{KrF6}$ are far less stable than $\ce{KrF2}$. The quantum chemical calculation confirmed this (Dixon 2007). The same reference said that they are stable, but have not been synthesized yet, and the synthesis would be very difficult.


For radon it is a different problem. Briefly, there might be more fluoride compounds of radon than xenon, but the synthesis is difficult to be confirmed.

Even though radon is less inert than xenon and its compounds were predicted and calculated to be more chemically stable than xenon compounds (Chernick 1962, Thayer 2005), it is difficult to study. Radon is is rare and radioactive (half-life of 3.8 days). Studying chemical compounds in trace amount is a difficult task. There are claims for discoveries of $\ce{RnF4}$ and $\ce{RnF6}$ (also in the above references), but confirming that you have synthesized new compounds in trace amount is extremely difficult because the evidence is not obvious. $\ce{RnF8}$ was calculated to be unstable, but should be more possible than $\ce{XeF8}$.


References:

1 David A. Dixon, Tsang-Hsiu Wang, Daniel J. Grant, Kirk A. Peterson, Karl O. Christe, Gary J. Schrobilgen, "Heats of formation of krypton fluorides and stability predictions for $\ce{KrF4}$ and KrF6$\ce{KrF46}$ from high level electronic structure calculations," Inorganic Chemistry 2007, 46(23), 10016-10021 (https://doi.org/10.1021/ic701313h).

2 C. L. Chernick, H. H. Claassen, P. R. Fields, H. H. Hyman, J. G. Malm, W. M. Manning, M. S. Matheson, L. A. Quarterman, F. Schreiner, H. H. Selig, I. Sheft, S. Siegel, E. N. Sloth, L. Stein, M. H. Studier, J. L. Weeks, M. H. Zirin, "Fluorine Compounds of Xenon and Radon," Science 1962, 138(3537), 136-138 (DOI: 10.1126/science.138.3537.136).

3 John S. Thayer, "Relativistic Effects and the Chemistry of the Heaviest Main-Group Elements," Journal of Chemical Education 2005, 82(11), 1721-1727 (https://doi.org/10.1021/ed082p1721).

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    $\begingroup$ Another reason why the fluorides of the lighter noble gasses are less stable is, that the elements are smaller, which leads to a smaller fluorine-fluorine distances and higher repulsion between the non-bonding electrons (Pauli repulsion). $\endgroup$
    – Martin - マーチン ♦
    Commented Jun 30, 2015 at 13:35
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Most of the noble gases compounds involve xenon, fluorine and oxygen. The only known binary compounds of noble gases are fluorides and oxides of xenon and to a lesser extent of krypton. For example, $\ce{XeF2, KrF2, XeF4, XeF6, XeO3, XeOF2,}$ etc. but not $\ce{NeF4}$.

Noble gases form compounds with fluorine and oxygen only because fluorine and oxygen are the most electronegative elements. Heavier noble gases (xenon and krypton) are able to form compounds with other elements because xenon and krypton, both have low ionisation enthalpies as compared to helium and neon. The noble gas elements do not react with other elements as they are less electronegative and are not able to attract the lone pair of electrons from noble elements. Fluorine and oxygen are more electronegative elements. They are strong oxidizing agents. They can easily attract the lone pair of noble gas elements.

See also Why are inert gas (especially xenon) compounds powerful oxidizing agents? and Why Aren't Chlorides Of The Noble Gases As Prevalent As Their Fluorides?

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