Radon (Rn). Diagram of the nuclear composition and electron configuration of an atom of radon-222 (atomic number: 86), the most stable isotope of this element. The nucleus consists of 86 protons (red) and 136 neutrons (yellow). 86 electrons (white) bind to the nucleus, successively occupying available electron shells (rings). Radon is a noble gas in group 18, period 6, and the p-block of the periodic table. It is the only known radioactive gas. Radon-222, produced from the decay chain of uranium or thorium ores, has a half-life of just under 4 days, emitting an alpha particle from its unstable nucleus to form polonium-218