Praseodymium is a chemical element; it has symbol Pr and atomic number 59. It is the third member of the lanthanide series and is considered one of the rare-earth metals. It 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 sixth-most abundant rare-earth element and fourth-most abundant lanthanide, 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 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 Ancient Greek (), meaning 'leek-green', and () '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 at low temperatures. The 0, +1, and +2 oxidation states are rarely found. 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.
Physical properties Praseodymium is the third member of the lanthanide series, and a member of the rare-earth metals. 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. Praseodymium is calculated to have a very large atomic radius; with a radius of 247 pm, barium, rubidium and caesium are larger. However, observationally, it is usually 185 pm.
Neutral praseodymium's 59 electrons are arranged in the configuration [Xe]4f36s2. Like most other lanthanides, praseodymium usually uses only three electrons as valence electrons, as the remaining 4f electrons are too strongly bound to engage in bonding: 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 penetration increases with higher ionic charge. Even so, praseodymium can in some compounds lose a fourth valence electron because it is 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.Greenwood and Earnshaw, pp. 1232–8
Similarly to the other early lanthanides, praseodymium has a double hexagonal close-packed crystal structure at room temperature, called the alpha phase (α-Pr). At it transforms to a different allotrope that has a body-centered cubic structure (β-Pr), and it melts at .
Praseodymium, like all of the lanthanides, 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.
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 :
This may be reduced to praseodymium(III) oxide with hydrogen gas. Praseodymium(IV) oxide, , 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 of in boiling acetic acid. The reactivity of praseodymium conforms to periodic trends, as it is one of the first and thus one of the largest lanthanides. At 1000 °C, many praseodymium oxides with composition PrO2−x exist as disordered, nonstoichiometric phases with 0 < x < 0.25, but at 400–700 °C the oxide defects are instead ordered, creating phases of the general formula with n = 4, 7, 9, 10, 11, 12, and ∞. These phases PrOy are sometimes labelled α and β′ (nonstoichiometric), β (y = 1.833), δ (1.818), ε (1.8), ζ (1.778), ι (1.714), θ, and σ.
Praseodymium is an electropositive element and reacts slowly with cold water and quite quickly with hot water to form praseodymium(III) hydroxide:
Praseodymium metal reacts with all the stable halogens to form trihalides: [green] [green] [green]
tetrafluoride, PrF4, is also known, and is produced by reacting a mixture of sodium fluoride and praseodymium(III) fluoride with fluorine gas, producing , following which sodium fluoride is removed from the reaction mixture with liquid hydrogen fluoride. Additionally, praseodymium forms a bronze diiodide; like the diiodides of lanthanum, cerium, and gadolinium, it is a praseodymium(III) electride compound.
Praseodymium dissolves readily in dilute sulfuric acid to form solutions containing the chartreuse ions, which exist as complexes:
Dissolving praseodymium(IV) compounds in water does not result in solutions containing the yellow ions; because of the high positive standard reduction potential of the/ couple at +3.2 V, these ions are unstable in aqueous solution, oxidising water and being reduced to . The value for the /Pr couple is −2.35 V. However, in highly basic aqueous media, ions can be generated by oxidation with ozone.
Praseodymium(V) has been observed by matrix isolation (in 2016) and in the bulk state (in 2025). The existence of praseodymium in its +5 oxidation state (with the stable electron configuration of the preceding noble gas xenon) under noble-gas matrix isolation conditions was reported in 2016. The species assigned to the +5 state were identified as , its and Ar adducts, and . Further, in 2025, a neutral compound , formally Pr(V) but with an inverted ligand field, was isolated and characterized crystallographically at low temperatures.
Organopraseodymium compounds
Organopraseodymium compounds are very similar to those of the other lanthanides, as they all share an inability to undergo π backbonding. They are thus mostly restricted to the mostly ionic cyclopentadienides (isostructural with those of lanthanum) and the σ-bonded simple alkyls and aryls, some of which may be polymeric. The coordination chemistry of praseodymium is largely that of the large, electropositive ion, and is thus largely similar to those of the other early lanthanides , , and . For instance, like lanthanum, cerium, and neodymium, praseodymium nitrates form both 4:3 and 1:1 complexes with 18-crown-6, whereas the middle lanthanides from promethium to gadolinium can only form the 4:3 complex and the later lanthanides from terbium to lutetium cannot successfully coordinate to all the ligands. Such praseodymium complexes have high but uncertain coordination numbers and poorly defined stereochemistry, with exceptions resulting from exceptionally bulky ligands such as the tricoordinate . There are also a few mixed oxides and fluorides involving praseodymium(IV), but it does not have an appreciable coordination chemistry in this oxidation state like its neighbour cerium. However, the first example of a molecular complex of praseodymium(IV) has recently been reported.
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. Thirty-eight 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 13.57 days. Both 141Pr and 143Pr occur as fission products of uranium (as do heavier isotopes). 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.
History
In 1751, the Swedish mineralogist Axel Fredrik Cronstedt discovered a heavy mineral from the mine at Bastnäs, later named cerite. Thirty years later, the fifteen-year-old Wilhelm Hisinger, from the family owning the mine, sent a sample of it to Carl Scheele, who did not find any new elements within. In 1803, after Hisinger had become an ironmaster, he returned to the mineral with Jöns Jacob Berzelius and isolated a new oxide, which they named ceria after the dwarf planet Ceres, which had been discovered two years earlier.Emsley, pp. 120–5 Ceria was simultaneously and independently isolated in Germany by Martin Heinrich Klaproth.Greenwood and Earnshaw, p. 1424 Between 1839 and 1843, ceria was shown to be a mixture of oxides by the Swedish surgeon and chemist Carl Gustaf Mosander, who lived in the same house as Berzelius; he separated out two other oxides, which he named lanthana and didymia. He partially decomposed a sample of cerium nitrate by roasting it in air and then treating the resulting oxide with dilute nitric acid. The metals that formed these oxides were thus named lanthanum and didymium.(Berzelius) (1839) "Nouveau métal" (New metal), Comptes rendus, 8 : 356–357. From p. 356: "L'oxide de cérium, extrait de la cérite par la procédé ordinaire, contient à peu près les deux cinquièmes de son poids de l'oxide du nouveau métal qui ne change que peu les propriétés du cérium, et qui s'y tient pour ainsi dire caché. Cette raison a engagé M. Mosander à donner au nouveau métal le nom de Lantane." (The oxide of cerium, extracted from cerite by the usual procedure, contains almost two fifths of its weight in the oxide of the new metal, which differs only slightly from the properties of cerium, and which is held in it so to speak "hidden". This reason motivated Mr. Mosander to give to the new metal the name Lantane.)(Berzelius) (1839) "Latanium — a new metal," Philosophical Magazine, new series, 14 : 390–391.
While lanthanum turned out to be a pure element, didymium was not and turned out to be only a mixture of all the stable early lanthanides from praseodymium to europium, as had been suspected by Marc Delafontaine after spectroscopic analysis, though he lacked the time to pursue its separation into its constituents. The heavy pair of samarium and europium were only removed in 1879 by Paul-Émile Lecoq de Boisbaudran and it was not until 1885 that Carl Auer von Welsbach separated didymium into praseodymium and neodymium. Von Welsbach confirmed the separation by spectroscopic analysis, but the products were of relatively low purity. Since neodymium was a larger constituent of didymium than praseodymium, it kept the old name with disambiguation, while praseodymium was distinguished by the leek-green colour of its salts (Greek πρασιος, "leek green").Greenwood and Earnshaw, p. 1229–32 The composite nature of didymium had previously been suggested in 1882 by Bohuslav Brauner, who did not experimentally pursue its separation.
Occurrence