Protactinium

thorium ← protactinium → uranium Pr ↑ Pa ↓ —

Atomic properties Atomic number 91 Standard atomic weight 231.035 88(2) Electron configuration [Rn] 5f2 6d1 7s2 Physical properties Melting point 1565(19) °C (1838 K) Boiling point ca. 4227 °C (4500 K) Density 15.37(8) g cm−3 Electric resistivity 19.1 × 10−6 Ω cm Chemical properties Electronegativity 1.5 (Pauling) Ionization energy 5.89(12) eV 568(12) kJ mol−1 Atomic radii Covalent radius 200 pm Metallic radius 163 pm Ionic radius 92 pm (Pa5+, Oh) Thermodynamic properties Enthalpy change of fusion 16.7 kJ mol−1 Enthalpy change of vaporization 481 kJ mol−1 Miscellaneous CAS number 7440-13-3 Where appropriate, and unless otherwise stated, data are given for 100 kPa (1 bar) and 298.15 K (25 °C).

Protactinium (symbol: Pa) is a chemical element, a member of the actinoid series. Although all isotopes of protactinium are unstable, protactinium-231 (t_½ = 3.276(11) × 10⁴ a) is formed as a decay product of uranium-235 (t_½ = 704(1) × 10⁶ a) and so small amounts of protactinium are present in all samples of natural uranium.

The existence of an element between thorium and uranium was predicted in 1872 by Mendeleev, who named the putative element eka-tantalum. The chemistry of protactinium does resemble that of niobium and tantalum, with a preponderence of the +5 oxidation state, and periodic tables from before the Second World War often placed protactinium as a transition metal.

Notes and references

Further reading

• Brown, D.; Maddock, A. G. Protactinium. Q. Rev. Chem. Soc. 1963, 17, 289–341. DOI: 10.1039/QR9631700289.

External links

• WebElements