Praseodymium is a soft, silvery lanthanide metal. It is reactive, forms a green oxide surface, and is used in strong permanent magnets, specialty glasses, and alloys.
The ground-state configuration is [Xe] 4f3 6s2 (with very small 5d occupancy in the atom). The presence of three 4f electrons leads to the common Pr(III) state (4f2 in compounds due to electron removal) and, in oxidizing environments, accessible Pr(IV) chemistry in certain oxides and fluorides.
+3 is dominant (e.g., PrCl3, Pr2O3). Under strong oxidizing conditions, +4 occurs in mixed-valence oxides (e.g., Pr6O11) and in some fluoride complexes (e.g., PrF4). Pr(II) is rare and unstable in aqueous media.
Fresh Pr rapidly forms a green surface due to a mixture of oxides and oxyhydroxides, commonly written as Pr2O3 and mixed-valence Pr6O11. The latter contains both Pr(III) and Pr(IV), which contributes to the characteristic color.
Reactions are typical of reactive lanthanides:
\(\mathrm{4\,Pr(s) + 3\,O_2(g) \rightarrow 2\,Pr_2O_3(s)}\)
\(\mathrm{2\,Pr(s) + 6\,H_2O(l) \rightarrow 2\,Pr(OH)_3(s) + 3\,H_2(g)}\)
Finely divided Pr can oxidize vigorously; store metal under oil or inert gas.
Key uses include:
Didymium was once thought to be an element but is actually a mixture primarily of Pr and Nd rare-earths. Today, separated Pr/ Nd oxides are recombined in controlled ratios for glass filters (e.g., for blowtorching and calibration standards).
After mineral cracking (acid/alkali), the rare-earth mix is separated by solvent extraction and ion exchange that exploit subtle differences in Ln3+ ionic radii and complexation. Pr is then precipitated/calcined to Pr6O11 or reduced to metal.
The lanthanide contraction is the steady decrease in Ln3+ ionic radii from La to Lu due to poor 4f shielding. Pr3+ (early in the series) has a relatively larger radius, allowing somewhat higher coordination numbers and slightly different complex stabilities compared to later lanthanides.
Yes. Pr3+ exhibits characteristic f–f emissions (narrow lines) in suitable hosts. For example, in certain fluorides or phosphates, Pr3+ can emit in green–red regions upon UV/blue excitation, useful for specialty lighting and display phosphors.
Bulk metal has low acute toxicity, but fine powders can be reactive and irritating. Some soluble Pr salts may affect the respiratory tract and eyes. Use gloves, goggles, and ventilation; store metal under oil or inert atmosphere and keep away from oxidizers.
Mixed-valence behavior is typical, with Pr cycling between +3 and +4 in oxides:
\(\mathrm{3\,Pr_2O_3 + O_2 \rightarrow 2\,Pr_3O_7}\)
This reflects the formation of oxygen-rich, higher-valent phases (schematically written) that underpin catalytic and coloring properties.