Atomic number 59

Praseodymium

Pr
+ phase
− phase

Shells Orbitals 4f 6s

Drag to rotate; pinch or Ctrl + scroll to zoom. Arrow keys rotate, + − zoom, R resets.

59 electrons in 6 shells: 2, 8, 18, 21, 8, 2.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Lanthanide
Period: 6
Block: f
Standard atomic weight: 140.91(1) Da
Electron configuration: [Xe] 4f³ 6s²
Phase at 25 °C: solid
Melting point: 931 °C
Boiling point: 3130 °C
Density: 6.77 g/cm³
Electronegativity (Pauling): 1.13
Discovered: 1885, Carl Auer von Welsbach

Neighbours in the table

Ce Nd

Two Greek words, prasios for green and didymos for twin, make up the name praseodymium. The green half refers to the element’s pale green salts, according to CIAAW, the atomic-weights commission of the IUPAC (the International Union of Pure and Applied Chemistry). The twin half points back to didymium, long taken for a single element. In 1885 the Austrian chemist Carl Auer von Welsbach split it in two, according to CIAAW. The halves were praseodymia and neodymia, each giving salts of its own colour, Los Alamos records.

The metal took almost half a century longer, and relatively pure praseodymium was first prepared in 1931, according to Los Alamos. It is soft and silvery, and also malleable and ductile, able to be hammered into sheets or drawn into wire. Those two traits come from metallic bonding, in Chem1’s account: each ion sits in a fluid of electrons on every side, so the bonding has no preferred direction.

In air the metal resists corrosion somewhat better than europium, lanthanum, cerium or neodymium, Los Alamos reports. It still grows a green oxide coating, and that coating falls away when exposed to air. For that reason, Los Alamos notes, the metal is kept in plastic or under oil.

Where it is found

Each kilogram of the Earth’s crust holds an estimated 9.2 milligrams of praseodymium in Jefferson Lab’s figures, and a litre of seawater 6.4 × 10⁻⁷ milligrams. Every natural atom of it is praseodymium-141, the element’s single stable isotope in Jefferson Lab’s figures.

Praseodymium occurs with the other rare earths in a variety of minerals. Monazite and bastnasite are the two leading commercial sources of the rare-earth metals, Los Alamos notes. The element is recovered mostly by ion exchange from monazite sand, a phosphate with the formula (Ce, La, Th, Nd, Y)PO₄, Jefferson Lab reports. Among several ways to make the metal, calcium can reduce its anhydrous chloride or fluoride, Los Alamos adds.

California turned out compounds of praseodymium and neodymium in both 2023 and 2024, by the account of the US Geological Survey. Ion exchange and solvent extraction have made the rare earths, praseodymium among them, far easier and cheaper to isolate, Los Alamos notes. World mine output of the whole group reached an estimated 390,000 tons of rare-earth oxide equivalent in 2024.

Why it matters

Praseodymium’s chief use, in Jefferson Lab’s account, is as an alloying partner for magnesium, making high-strength metals for aircraft engines. Misch metal, the alloy that lighter flints are made from, contains about 5 per cent praseodymium, according to Los Alamos. The element also appears in neodymium-praseodymium oxide, among the US government stockpile’s potential purchases for fiscal 2024 and 2025, the US Geological Survey reports.

Mixed with certain other materials, praseodymium gives glass an intense yellow that Los Alamos calls unusually clean. Salts of the element colour both glasses and enamels. It is also one ingredient of didymium glass, the tint in goggles worn by welders and, Jefferson Lab adds, by glass blowers.

Doped into fibre-optic cable, praseodymium works as a signal amplifier, Jefferson Lab reports. It has also served as core material in the carbon arcs that lit film studios and ran cinema projectors, Los Alamos notes.

Praseodymium-140, whose half-life is 3.39 minutes in the Atomic Mass Data Center’s table on PubChem’s record, has been planned as an imaging agent, in IUPAC’s isotope notes. A radionuclide generator, a device designed to supply radioactive metal complexes for imaging, has been built around neodymium-140 and praseodymium-140. It is meant to provide praseodymium-140 complexes as a tracer for PET (positron emission tomography) scans.

Properties

Properties of praseodymium Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	59
Standard atomic weight	140.91(1) Da	Da	IUPAC/CIAAW 2024
Melting point	931 °C	°C	PubChem: 1204 K
Boiling point	3130 °C	°C	Los Alamos: 3130 °C
Density	6.77 g/cm³	g/cm³
Electronegativity (Pauling)	1.13
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f³
Phase at 25 °C	solid

Safety

Praseodymium metal is a flammable, self-heating solid that irritates skin and eyes in just over half of 247 company reports under the Globally Harmonized System (GHS). That classification carries the signal word Danger. In 28.3% of reports it can catch fire by itself on contact with air, while 18.2% find no hazard. Classification and handling guidance are in the PubChem Laboratory Chemical Safety Summary for praseodymium.

Questions

Is praseodymium radioactive?

Natural praseodymium is not: every atom of it is praseodymium-141, the element's one stable isotope. Two radioactive isotopes have been proposed for medicine. Praseodymium-142, produced by neutron activation of praseodymium-141, has a half-life of 19.12 hours. Its beta radiation reaches only a few millimetres into tissue, in PubChem's isotope notes, and it has been proposed for microsphere and eye plaque brachytherapy, radiation treatment from a source placed inside or against the body.

What is the melting point of praseodymium?

Praseodymium melts at 931 °C and boils at 3520 °C in Jefferson Lab's figures on PubChem's record; Los Alamos gives 3130 °C for boiling. Its oxide, Pr₂O₃, is among the most refractory substances known, Los Alamos notes, meaning it withstands very high temperatures.

How did praseodymium help find promethium?

In 1941 researchers at Ohio State University aimed neutrons, deuterons and alpha particles at targets of neodymium and praseodymium, Los Alamos recounts. The targets gave off several new radioactivities, most likely from element 61, later named promethium. Chemical proof was missing at the time, because separating one rare earth from another was still very difficult.

How was praseodymium separated from didymium?

Carl Gustav Mosander extracted didymia from lanthana in 1841, and in 1879 Lecoq de Boisbaudran drew samaria, a new earth, out of didymia taken from the mineral samarskite, Los Alamos notes. Auer von Welsbach finished the job in 1885, splitting what remained by fractionating ammonium didymium nitrate over and over until praseodymia and neodymia stood apart.