Promethium compounds are compounds containing the element promethium, which normally take the +3 oxidation state. Promethium belongs to the cerium group of lanthanides and is chemically very similar to the neighboring elements.[1] Because of its instability, chemical studies of promethium are incomplete. Even though a few compounds have been synthesized, they are not fully studied; in general, they tend to be pink or red in color.[2][3] Treatment of acidic solutions containing Pm3+ ions with ammonia results in a gelatinous light-brown sediment of hydroxide, Pm(OH)3, which is insoluble in water.[4] When dissolved in hydrochloric acid, a water-soluble yellow salt, PmCl3, is produced;[4] similarly, when dissolved in nitric acid, a nitrate results, Pm(NO3)3. The latter is also well-soluble; when dried, it forms pink crystals, similar to Nd(NO3)3.[4] The electron configuration for Pm3+ is [Xe] 4f4, and the color of the ion is pink. The ground state term symbol is 5I4.[5] The sulfate is slightly soluble, like the other cerium group sulfates. Cell parameters have been calculated for its octahydrate; they lead to conclusion that the density of Pm2(SO4)3·8 H2O is 2.86 g/cm3.[6] The oxalate, Pm2(C2O4)3·10 H2O, has the lowest solubility of all lanthanide oxalates.[7]

Unlike the nitrate, the oxide is similar to the corresponding samarium salt and not the neodymium salt. As-synthesized, e.g. by heating the oxalate, it is a white or lavender-colored powder with disordered structure.[4] This powder crystallizes in a cubic lattice upon heating to 600 °C. Further annealing at 800 °C and then at 1750 °C irreversibly transforms it to monoclinic and hexagonal phases, respectively, and the last two phases can be interconverted by adjusting the annealing time and temperature.[8]

Formula symmetry space group No Pearson symbol a (pm) b (pm) c (pm) Z density,
g/cm3
α-Pm dhcp[9][10] P63/mmc 194 hP4 365 365 1165 4 7.26
β-Pm bcc[10] Fm3m 225 cF4 410 410 410 4 6.99
Pm2O3 cubic[8] Ia3 206 cI80 1099 1099 1099 16 6.77
Pm2O3 monoclinic[8] C2/m 12 mS30 1422 365 891 6 7.40
Pm2O3 hexagonal[8] P3m1 164 hP5 380.2 380.2 595.4 1 7.53

Promethium forms only one stable oxidation state, +3, in the form of ions; this is in line with other lanthanides. According to its position in the periodic table, the element cannot be expected to form stable +4 or +2 oxidation states; treating chemical compounds containing Pm3+ ions with strong oxidizing or reducing agents showed that the ion is not easily oxidized or reduced.[1]

Promethium halides[11]
Formula color coordination
number 		symmetry space group No Pearson symbol m.p. (°C)
PmF3 Purple-pink 11 hexagonal P3c1 165 hP24 1338
PmCl3 Lavender 9 hexagonal P63/mc 176 hP8 655
PmBr3 Red 8 orthorhombic Cmcm 63 oS16 624
α-PmI3 Red 8 orthorhombic Cmcm 63 oS16 α→β
β-PmI3 Red 6 rhombohedral R3 148 hR24 695

Bibliography

References

  1. ^ a b Lavruk︠h︡ina & Pozdni︠a︡kov 1970, p. 120.
  2. ^ Emsley, J. (2011). Nature's Building Blocks: An A-Z Guide to the Elements. OUP Oxford. p. 429. ISBN 978-0-19-257046-8.
  3. ^ promethium. Encyclopædia Britannica Online
  4. ^ a b c d Lavruk︠h︡ina & Pozdni︠a︡kov 1970, p. 121.
  5. ^ Aspinall, H. C. (2001). Chemistry of the f-block elements. Gordon & Breach. p. 34, Table 2.1. ISBN 978-9056993337.
  6. ^ Lavruk︠h︡ina & Pozdni︠a︡kov 1970, p. 122.
  7. ^ Lavruk︠h︡ina & Pozdni︠a︡kov 1970, p. 123.
  8. ^ a b c d Chikalla, T. D.; McNeilly, C. E.; Roberts, F. P. (1972). "Polymorphic Modifications of Pm2O3". Journal of the American Ceramic Society. 55 (8): 428. doi:10.1111/j.1151-2916.1972.tb11329.x.
  9. ^ Pallmer, P. G.; Chikalla, T. D. (1971). "The crystal structure of promethium". Journal of the Less Common Metals. 24 (3): 233. doi:10.1016/0022-5088(71)90101-9.
  10. ^ a b Gschneidner Jr., K.A. (2005). "Physical Properties of the rare earth metals" (PDF). In Lide, D. R. (ed.). CRC Handbook of Chemistry and Physics (86th ed.). Boca Raton, FL: CRC Press. ISBN 978-0-8493-0486-6. Archived from the original (PDF) on 2012-09-18. Retrieved 2012-06-20.
  11. ^ Cotton, Simon (2006). Lanthanide And Actinide Chemistry. John Wiley & Sons. p. 117. ISBN 978-0-470-01006-8.
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