penetrate the most through the inert xenon core of electrons to the nucleus, followed by 5d and 6s, and this increases with higher ionic charge. Praseodymium nevertheless can continue losing a fourth and even occasionally a fifth valence electron because it comes very early in the lanthanide series, where the nuclear charge is still low enough and the 4f subshell energy high enough to allow the removal of further valence electrons. Thus, similarly to the other early trivalent lanthanides, praseodymium has a double hexagonal close-packed crystal structure at room temperature. At about 560 °C, it transitions to a face-centered cubic structure, and a body-centered cubic structure appears shortly before the melting point of 935 °C. Praseodymium, like all of the lanthanides (except lanthanum, ytterbium, and lutetium, which have no unpaired 4f electrons), is paramagnetic at room temperature. Unlike some other rare-earth metals, which show antiferromagnetic or ferromagnetic ordering at low temperatures, praseodymium is paramagnetic at all temperatures above 1 K. Isotopes Praseodymium has only one stable and naturally occurring isotope, 141Pr. It is thus a mononuclidic and monoisotopic element, and its standard atomic weight can be determined with high precision as it is a constant of nature. This isotope has 82 neutrons, which is a magic number that confers additional stability. This isotope is produced in stars through the s- and r-processes (slow and rapid neutron capture, respectively). 38 other radioisotopes have been synthesized. All of these isotopes have half-lives under a day (and most under a minute), with the single exception of 143Pr with a half-life of 13.6 days. Both 143Pr and 141Pr occur as fission products of uranium. The primary decay mode of isotopes lighter than 141Pr is positron emission or electron capture to isotopes of cerium, while that of heavier isotopes is beta decay to isotopes of neodymium. Chemical properties Praseodymium metal tarnishes slowly in air, forming a spalling green oxide layer like iron rust; a centimetre-sized sample of praseodymium metal corrodes completely in about a year. It burns readily at 150 °C to form praseodymium(III,IV) oxide, a nonstoichiometric compound approximating to Pr6O11: 12 Pr + 11 O2 → 2 Pr6O11 This may be reduced to praseodymium(III) oxide (Pr2O3) with hydrogen gas. Praseodymium(IV) oxide, PrO2, is the most oxidised product of the combustion of praseodymium and can be obtained by either reaction of praseodymium metal with pure oxygen at 400 °C and 282 bar or by disproportionation