Praseodymium

Praseodymium is a chemical element with the symbol Pr and atomic number 59. It is the third member of the lanthanide series and is traditionally considered to be one of the rare-earth metals. Praseodymium is a soft, silvery, malleable and ductile metal, valued for its magnetic, electrical, chemical, and optical properties. It is too reactive to be found in native form, and pure praseodymium metal slowly develops a green oxide coating when exposed to air. Praseodymium always occurs naturally together with the other rare-earth metals. It is the fourth most common rare-earth element, making up 9.1 parts per million of the Earth's crust, an abundance similar to that of boron. In 1841, Swedish chemist Carl Gustav Mosander extracted a rare-earth oxide residue he called didymium from a residue he called 'lanthana', in turn separated from cerium salts. In 1885, the Austrian chemist Baron Carl Auer von Welsbach separated didymium into two elements that gave salts of different colours, which he named praseodymium and neodymium. The name praseodymium comes from the Greek prasinos (πράσινος), meaning 'green', and didymos (δίδυμος), 'twin'. Like most rare-earth elements, praseodymium most readily forms the +3 oxidation state, which is the only stable state in aqueous solution, although the +4 oxidation state is known in some solid compounds and, uniquely among the lanthanides, the +5 oxidation state is attainable in matrix-isolation conditions. Aqueous praseodymium ions are yellowish-green, and similarly praseodymium results in various shades of yellow-green when incorporated into glasses. Many of praseodymium's industrial uses involve its ability to filter yellow light from light sources. Praseodymium is the third member of the lanthanide series. In the periodic table, it appears between the lanthanides cerium to its left and neodymium to its right, and above the actinide protactinium. It is a ductile metal with a hardness comparable to that of silver. Its 59 electrons are arranged in the configuration 4f36s2; theoretically, all five outer electrons can act as valence electrons, but the use of all five requires extreme conditions and normally, praseodymium only gives up three or sometimes four electrons in its compounds. Praseodymium is the first of the lanthanides to have an electron configuration conforming to the Aufbau principle, which predicts the 4f orbitals to have a lower energy level than the 5d orbitals; this does not hold for lanthanum and cerium, because the sudden contraction of the 4f orbitals does not happen until after lanthanum, and is not strong enough at cerium to avoid occupying the 5d subshell. Nevertheless, solid praseodymium takes on the 4f25d16s2 configuration, with one electron in the 5d subshell like all the other trivalent lanthanides (all but europium and ytterbium, which are divalent in the metallic state). Like most lanthanides, praseodymium usually only uses three electrons as valence electrons, as afterwards the remaining 4f electrons are too strongly bound: this is because the 4f orbitals 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. Praseodymium has only one stable and naturally occurring isotope, 141Pr. It is thus a mononuclidic element, and its standard atomic weight can be determined with high precision as it is a constant of nature. This isotope has 82 neutrons, 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). All other praseodymium 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 inverse beta decay or electron capture to isotopes of cerium, while that of heavier isotopes is beta decay to isotopes of neodymium.