Why don't radon and krypton bond easily?

A common chemistry question asks which statement best explains why radon and krypton do not bond easily with other elements. The answer is the one about valence electrons: each has a complete set of eight in its outer shell. Answers about atomic mass do not explain it: radon is far heavier than krypton, 222.01758 against 83.80, yet both behave alike.

In each configuration, the noble gas in brackets is the core: argon's 18 electrons for krypton, xenon's 54 for radon. What follows fills the next shell to eight outer electrons. Xenon, between them in the group, follows the same pattern with 5s2 5p6.

Drawn as Lewis dot structures, Kr and Rn look alike: the symbol with eight dots in four pairs. See how many valence electrons radon has.

Radon is essentially inert, but not completely. Fluorine has been reported to react with radon, forming a fluoride, and radon fluoride is the only confirmed compound of radon. Its oxidation states are listed as 6, 2 and 0.

Radon's first ionization energy is lower than krypton's, 10.745 eV against 14.000 eV, so by that measure radon holds its outer electrons less tightly than krypton does.

Since radon is a noble gas, it releases from any chemical bonds that attach it, and it may travel far enough to reach groundwater or the air. It comes from the natural decay of uranium and radium in nearly all rocks and soils and can move up from the ground into buildings through openings in floors or walls in contact with the ground.