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I read this statement that:

Xenon is the most soluble noble gas in water.

My first doubt is:
Why does the solubility of noble gases in water increase down the group?

Even if this is the trend, why is Radon not considered most soluble?

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Wikipedia gives the following solubilities for the noble gases in water at $25^\circ \text C$ in $\mathrm{cm^3~kg^{-1}}$:

$\mathrm{He: 8.61~~~Ne: 10.5~~~ Ar: 33.6~~~ Kr: 59.4 ~~~Xe: 108.1 ~~~Rn: 230}$

Why does solubility of noble gases increase down the group?

The intermolecular interactions of noble gases are in the form of London dispersion forces, which are strongly influenced by the polarizability of the atom which increases as the atomic radius increases. This can be thought of as resulting from the electrons being 'less tightly bound' to the nucleus and the atomic radius increases. Atomic radius increases down the group therefore solubility also increases. For more on polarizability see here.

Why is radon not the most soluble noble gas?

Radon (and other heavy radioactive elements) are often ignored when talking about trends because they are uncommon. Radon is in fact the most soluble noble gas but the half lives of its isotopes are on the order of hours or days so it is not really very useful.

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The solubility of radon in water has long been studied, and the Weigel equation in the name of the equation for radon's solubility in water as a function of temperature, at atmospheric pressure.

[Rn(aq)]/[Rn(g)] = 0.105 + 0.405 * exp(-0.0502*T)

Near room temperature, the concentration of radon in the gas phase will be about 4 times the amount in the liquid phase.

Radon is the most soluble noble gas.

See Radon Solubility In Water As A Function Of Salinity And Temperature by Eric B. Lieberman for more information.

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The solubility in water is due to Dipole - induced dipole interaction. The polarizing water molecules distort their electrons with the increase in atomic size.Therefore increase in interaction or Atomic size. i.e. increase in solubility.

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    $\begingroup$ ‘The water molecules distort their electrons with the increase in atomic size’ — Water molecules are always the same size. Clarify please. $\endgroup$ – Jan Nov 23 '15 at 13:49
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    $\begingroup$ Also welcome to chemistry.stackexchange.com. Feel free to take a tour of the site. You can edit your post at any time with the link in the bottom left. For any questions about the site itself and how it works, visit the help center. $\endgroup$ – Jan Nov 23 '15 at 13:50

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