Radon

Radon (Rn) is a heavy, radioactive noble gas with an atomic number of 86. It is located below Xe in the periodic table, and between At and Fr. Radon is widely present throughout earth's crust and mantle since isotopes of Rn are produced in the decay chains of all three primordial actinides, ²³⁸U, ²³⁵U, and ²³²Th. It's principal usefulness lies in profitably scaring people about its dangers.

NUCLEAR PROPERTIES

Around 105 isotopes of radon have been predicted, from the neutron dripline near ²⁸⁰Rn to ¹⁷⁸Rn, of which 37 isotopes ranging from ²³¹Rn down to ¹⁹⁵Rn have been observed (as well as 12 isomers). These fall into three groups. From the dripline down to ²²³Rn, isotopes decay exclusively by beta emission with half-lives ranging from near 0.001 sec at the dripline to12 sec at ²²⁹Rn and peaking at 107 min in ²²⁴Rn. A second band goes from ²¹¹Rn down to ¹⁷⁸Rn. With the exception of ¹⁷⁸Rn itself, which is predicted to decay by proton emission; all isotopes in the second band decay by alpha emission. Between ²¹¹Rn and ¹⁹⁸Rn, positron emission exists as an alternate decay mode. Positron emission branch ratio rises with A, becoming dominant for odd-A isotopes between ²⁰⁹Rn and ²⁰⁵Rn. Positron emission branch ratio falls at ²¹⁰Rn and ²¹¹Rn. Although partial half lives against alpha decay continue to rise up to ²¹¹Rn, positron emission partial half-lives rise faster, particularly in odd-A isotopes. This explains why alpha decay is dominant at ²¹¹Rn. That isotope has the second-longest half-life of any Rn isotope at 14.6 +/- 2 hrs (well ahead of 209Rn's 2.4 hrs).

The third band, which runs from ²²²Rn to ²¹²Rn, demonstrates the powerful effect of neutron shell closure at N = 126, which occurs at ²¹²Rn. Most radon isotopes in this band are predicted to be stable against beta decay and positive beta decay (positron emission or electron capture). ²²¹Rn decays mainly by beta emission (which is predicted), but all other isotopes in the band decay only by alpha emission. At the band's upper end, the pattern is normal for heavy isotopes of an even-Z actinide. What is different with Rn is half-life. ²²²Rn, with its 3.2835 +/- 3 d half-life, is the element's longest-lived isotope (about 6.3 times the half-life of ²¹¹Rn). Below ²²²Rn, alpha decay half-lives tail off rapidly, falling below 1 sec at ²¹⁸Rn, 1 ms at ²¹⁶Rn, and reaching a minimum of 0.27 us (27 ns) at ²¹⁴Rn. Neutron count at ²¹⁴Rn is 128, two above the shell closure at N = 126. which means energy released by alpha decay will be large. Alpha decay should be very rapid, and is. The same is true of ²¹⁵Rn to ²²²Rn, although alpha decay becomes slower as A increases and the number of decays needed to reach stability at or adjacent to N = 128 grows. Even at ²²²Rn instability remains strong enough to give that isotope a surprisingly short half-live, considering its even-Z neighbors.

Heavy isotopes of Rn, down to ²¹⁷Rn, can be expected to form during a neutron star merger or a supernova in quantities on the same order of magnitude as Th or U via beta-decay chains from initially neutron-rich material in the right A range. ²¹⁶Rn is blocked by ²¹⁶Po and lighter isotopes are blocked by both At and Po, so cannot form in this way. A second process also contributes to production of Rn isotopes in the 222 >= A >= 219 band. ²²²Rn and ²¹⁸Rn descend from ²³⁸U, ²²⁰Rn descends from ²³²Th, and ²¹⁹Rn descends from ²³⁵U. Rather than becoming extinct, those isotopes come into equilibrium with their chain heads and remain that way as long as the chain head lasts. Equilibrium concentration ratio [X]/[chain head of X] can be computed to be [²²²Rn]/[²³⁸U] = 2.0E-12, [²²⁰Rn]/[²³²Th] = 1.3E-16, [²¹⁹Rn]/[²³⁵U] = 1.8E-16, and [²¹⁸Rn]/[²³⁹U] = 5.0E-26. In principle, ²²¹Rn forms via spontaneous fission of ²³⁸U and neutron capture by ²³⁵U and ²³⁶U, but [²²¹Rn]/[²³⁵U] will be orders of magnitude smaller than the negligible value for ²¹⁸Rn.

ATOMIC PROPERTIES

Radon has a closed valence shell, which makes it relatively inert. The key word is "relatively". Rn cannot form anions, but can be oxidized to both 2+ and 6+ states. For a noble gas, it has an extensive chemistry.

02-20-22