Praseodymium – facts and properties
- Symbol
- Pr
- Atomic number
- 59
- Atomic mass
- 140.908 u
- Period
- 6
- Category
- lanthanide
- Electron configuration
- [Xe] 4f³ 6s²
- Electronegativity
- 1.13
- Atomic radius
- 185 pm
- Valence electrons
- 5
- State at standard conditions
- solid
- Melting point
- 931 °C
- Boiling point
- 3520 °C
- Density
- 6.77 g/cm³
- Discovered
- 1885
Praseodymium is a soft, silvery metal with atomic number 59. Its name means green twin, from the Greek prasios, leek-green, and didymos, twin. It recalls that for decades what was taken for one element was really two: praseodymium and neodymium. Today praseodymium tints glass and ceramics yellow-green, filters yellow light in protective goggles and is added to strong magnets.
Position in the periodic table
Occurrence and extraction
Praseodymium accompanies cerium, lanthanum and neodymium as a minor constituent of the same ores. In the Earth's crust it is more common than tin. Because it is chemically very close to neodymium, the two are hard to separate. Industry therefore often sells a mixture of the two oxides, which can be used directly for magnets.
The story of its discovery is a story of the art of separation. Carl Gustaf Mosander, who had already extracted lanthanum from cerium compounds, believed in 1841 that he had found another element accompanying it, and called it didymium, the twin of lanthanum. Didymium salts were pink-violet and showed many narrow dark lines in their spectrum.
In 1885 the Austrian chemist Carl Auer von Welsbach showed that didymium is not an element but a mixture of two. He dissolved didymium ammonium nitrate in water and let it crystallise, redissolved the crystals and repeated this in many rounds. The two components piled up in different fractions, one forming green salts and the other pink ones. He named the green one praseodymium and the other neodymium.
Properties and reactions
Praseodymium is in period 6 and is the third lanthanide, with cerium to its left and neodymium to its right. Its electron configuration is [Xe] 4f³ 6s² and its electronegativity is 1.13. The metal has a density of 6.77 g/cm³, melts at 931 °C and boils at 3520 °C. It is soft and malleable, and somewhat more resistant to air than lanthanum or cerium. It does form a green oxide layer, though, which flakes off and exposes fresh metal.
In compounds praseodymium is mostly triply charged. The ions and many salts are yellow-green, hence the name. The stable oxide in air, Pr₆O₁₁, is dark brown to black and contains praseodymium(IV) as well as praseodymium(III).
The colour comes from the 4f electrons. They lie deep inside the atom and are shielded by the outer shells, so their surroundings hardly change their energies, and the ions absorb light in very narrow bands. That explains the lines in the spectrum of didymium and the pure colours of lanthanide ions. Praseodymium and its compounds are of low to moderate toxicity.
Uses in everyday life and technology
As a colourant, praseodymium gives strong tones. Praseodymium oxide tints glass yellow-green. In ceramics there is praseodymium yellow, zirconium silicate coloured with praseodymium, which turns glazes and tiles a bright yellow and survives firing heat.
A second glass application is didymium glass. It contains praseodymium and neodymium and swallows the yellow sodium line at about 589 nanometres. Glassblowers and gas welders look through such goggles at molten glass and metal without being dazzled by the glaring yellow light. They do not protect against the radiation of arc welding.
In technology, praseodymium is a smaller-volume additive in magnets. Neodymium-iron-boron magnets often contain some praseodymium in place of neodymium, and performance suffers little. In addition, praseodymium-doped optical fibres amplify light signals in data networks, and praseodymium is found in special alloys and laser materials.
Quick question
What is the atomic number of praseodymium (Pr)?
FAQ
What is didymium?
In the nineteenth century didymium was believed to be an element. It was a mixture of praseodymium and neodymium that Auer von Welsbach split by repeated crystallisation. The name lives on in didymium glass, which contains both elements.
Why are praseodymium salts green?
Triply charged praseodymium ions absorb light in narrow bands in the blue and yellow range. The rest of the light, mostly green, passes through. The 4f electrons responsible are shielded from outside, so the colour depends little on the surroundings.
What is praseodymium used for?
For yellow-green glass, yellow ceramic glazes, protective goggles made of didymium glass, as an additive in neodymium magnets and for light amplifiers in optical fibres.