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About Protactinium

Protactinium Bohr

Protactinium is a radioactive actinide with little known about its properties and no known commercial or industrial uses. It is a very scarce element, with only a few parts per million available even within uranium ore, the study of which led to its discovery. Protactinium’s decay chain leads to actinium, and thus its chemical element name refers to Proto-actinium. The most stable and abundant isotope of Protactinium (231Pa) has a half-life of 32,760 years and is itself produced through the decay chain of uranium (235U). Protactinium is an undesirable intermediate product in thorium-based nuclear reactors and must regularly be removed from the environment. Naturally-occuring protactinium isotopes in water are used along with uranium and thorium to date sediments and model geological processes.

Kasimir Fajans and Oswald Helmuth Göhring are credited with first identifying the protactinium element in 1913. However, full credit was never assigned for the discovery until 1917 when a stable isotope was discovered by Otto Hahn and Lise Meitner of Germany and Frederick Soddy and John Cranston of Great Britain. The first isolation of the element is credited to Aristid V. Grosse in 1934. Though many chemists have been acknowledged with having predicted, discovered, isolated, or produced the element since the year 1900, the element didn’t take its final form in the periodic table until 1949 when protoactinium was shortened to protactinium. Its discovery filled the last remaining gap in early versions of Mendeleev’s periodic table. In 1961, the United Kingdom Atomic Energy Authority produced 25 grams of protactinium for an estimated cost of one half-million dollars – which long stood as the world’s only supply of the element for scientific studies. However, Oak Ridge National Laboratory now produces and sells protactinium in small quantities.

Protactinium is a bright, shiny metal that forms compounds with the halogens and with hydrogen, though there are no known practical applications of any such compounds. Sitting between useful elements such as thorium and uranium on the periodic table, protactinium is itself not as useful as its neighbors. It is simply too scarce, radioactive, and toxic. Twenty-nine isotopes are known to exist, all of them radioactive. The isotopes all decay into actinium via alpha or beta chains depending on its isotope number. And though this element is rare, it can be found in minute amounts throughout Earth’s biosphere.

Protactinium Properties

Protactinium Bohr ModelProtactinium is a radioactive actinide group metal with the atomic symbol Pa, atomic number 91, and atomic weight 231. It decays by alpha-emission. It is a Block F, Group 3, Period 7 element. The number of electrons in each of Protactinium's shells is 2, 8, 18, 32, 20, 9, 2 and its electron configuration is [Rn] 5f2 6d1 7s2. It has an atomic radius of and a Van der Waals radius is In its elemental form protactinium's CAS number is 7440-13-3. It is found in the form of two isotopes: protactinium-231 and protactinium-234. Protactinium was first predicted by Dmitri Mendeleev in 1869 and first isolated by William Crookes in 1900. It was named by Otto Hahn and Lise Meitner  in 1917. The name Protactinium originates from the Greek word 'Porots' which means first.

Protactinium is both toxic and radioactive. Protactinium information, including technical data, safety data and its properties, research, applications and other useful facts are discussed below. Scientific facts such as the atomic structure, ionization energy, abundance on Earth, conductivity and thermal properties are included.

Symbol: Pa
Atomic Number: 91
Atomic Weight: 231
Element Category: Actinide
Group, Period, Block: n/a, 7, f
Color: silvery metallic
Other Names: Protoattinio, Protaktinium
Melting Point: 1572°C, 2861.6°F, 1845.15 K
Boiling Point: Unknown
Density: 15370  kg·m3
Liquid Density @ Melting Point: N/A
Density @ 20°C: 15.4 g/cm3
Density of Solid: 15370 kg·m3
Specific Heat: N/A
Superconductivity Temperature: 1.4 [or -271.7 °C (-457.1 °F)] K
Triple Point: N/A
Critical Point: N/A
Heat of Fusion (kJ·mol-1): 16.7
Heat of Vaporization (kJ·mol-1): 481
Heat of Atomization (kJ·mol-1): 607
Thermal Conductivity: 47 W·m-1·K-1
Thermal Expansion: N/A
Electrical Resistivity: (0 °C) 177 nΩ·m
Tensile Strength: N/A
Molar Heat Capacity: N/A
Young's Modulus: N/A
Shear Modulus: N/A
Bulk Modulus: N/A
Poisson Ratio: N/A
Mohs Hardness: N/A
Vickers Hardness: N/A
Brinell Hardness: N/A
Speed of Sound: N/A
Pauling Electronegativity: 1.5
Sanderson Electronegativity: N/A
Allred Rochow Electronegativity: 1.14
Mulliken-Jaffe Electronegativity: N/A
Allen Electronegativity: N/A
Pauling Electropositivity: 2.5
Reflectivity (%): N/A
Refractive Index: N/A
Electrons: 91
Protons: 91
Neutrons: 122
Electron Configuration: [Rn] 5f2 6d1 7s2
Atomic Radius: 163 pm
Atomic Radius,
non-bonded (Å):
Covalent Radius: 200 pm
Covalent Radius (Å): 1.84
Van der Waals Radius: 243 pm
Oxidation States: 5, 4, 3, 2, 0
Phase: Solid 
Crystal Structure: tetragonal
Magnetic Ordering: paramagnetic
Electron Affinity (kJ·mol-1) Unknown
1st Ionization Energy: 568 kJ·mol-1
2nd Ionization Energy: N/A
3rd Ionization Energy: N/A
CAS Number: 7440-13-3
EC Number: N/A
MDL Number: N/A
Beilstein Number: N/A
SMILES Identifier: [Pa]
InChI Identifier: InChI=1S/Pa
PubChem CID: 23945
ChemSpider ID: 22387
Earth - Total: N/A
Mercury - Total: N/A
Venus - Total: N/A
Earth - Seawater (Oceans), ppb by weight: 2.00E-16
Earth - Seawater (Oceans), ppb by atoms: 5.00E-18
Earth -  Crust (Crustal Rocks), ppb by weight: 0.00001
Earth -  Crust (Crustal Rocks), ppb by atoms: 9E-07
Sun - Total, ppb by weight: N/A
Sun - Total, ppb by atoms: N/A
Stream, ppb by weight: N/A
Stream, ppb by atoms: N/A
Meterorite (Carbonaceous), ppb by weight: N/A
Meterorite (Carbonaceous), ppb by atoms: N/A
Typical Human Body, ppb by weight: N/A
Typical Human Body, ppb by atom: N/A
Universe, ppb by weight: N/A
Universe, ppb by atom: N/A
Discovered By: William Crookes
Discovery Date: 1900
First Isolation: N/A

Protactinium Isotopes

Protactinium has three naturally occurring isotopes, none of them are stable.

Nuclide Isotopic Mass Half-Life Mode of Decay Nuclear Spin Magnetic Moment Binding Energy (MeV) Natural Abundance
(% by atom)
212Pa 212.02320(8) 8(5) ms [5.1(+61-19) ms] Unknown 7+# N/A 1591.95 -
213Pa 213.02111(8) 7(3) ms [5.3(+40-16) ms] a to 209Ac 9/2-# N/A 1600.02 -
214Pa 214.02092(8) 17(3) ms a to 210Ac N/A N/A 1608.1 -
215Pa 215.01919(9) 14(2) ms a to 211Ac 9/2-# N/A 1625.5 -
216Pa 216.01911(8) 105(12) ms a to 212Ac; ß+ to 216Th N/A N/A 1633.58 -
217Pa 217.01832(6) 3.48(9) ms a to 213Ac 9/2-# N/A 1641.66 -
218Pa 218.020042(26) 0.113(1) ms a to 214Ac N/A N/A 1640.42 -
219Pa 219.01988(6) 53(10) ns a to 215Ac; ß+ to 219Th 9/2- N/A 1657.81 -
220Pa 220.02188(6) 780(160) ns a to 226Ac 1-# N/A 1656.58 -
221Pa 221.02188(6) 4.9(8) µs a to 227Ac 9/2- N/A 1664.65 -
222Pa 222.02374(8)# 3.2(3) ms a to 228Ac N/A N/A 1672.73 -
223Pa 223.02396(8) 5.1(6) ms a to 229Ac; ß+ to 223Th N/A N/A 1680.81 -
224Pa 224.025626(17) 844(19) ms a to 220Ac; ß+ to 224Th 5-# N/A 1688.89 -
225Pa 225.02613(8) 1.7(2) s a to 221Ac 5/2-# N/A 1696.97 -
226Pa 226.027948(12) 1.8(2) min a to 222Ac; ß+ to 226Th N/A N/A 1705.05 -
227Pa 227.028805(8) 38.3(3) min a to 223Ac; EC to 227Th (5/2-) N/A 1713.13 -
228Pa 228.031051(5) 22(1) h EC to 228Th; a to 224Ac 3+ 3.5 1711.89 -
229Pa 229.0320968(30) 1.50(5) d EC to 229Th; a to 225Ac (5/2+) N/A 1719.97 -
230Pa 230.034541(4) 17.4(5) d EC to 230Th; a to 226Ac; ß- to 230U (2-) 2 1728.05 -
231Pa 231.0358840(24) 3.276(11)E+4 y a to 226Ac; SF 3/2- 2.01 1736.12 100
232Pa 232.038592(8) 1.31(2) d EC to 232Th; ß- to 232U (2-) N/A 1744.2 -
233Pa 233.0402473(23) 26.975(13) d ß- to 233U 3/2- 4 1742.97 -
234Pa 234.043308(5) 6.70(5) h ß- to 234U; SF 4+ N/A 1751.04 -
235Pa 235.04544(5) 24.44(11) min ß- to 235U (3/2-) N/A 1759.12 -
236Pa 236.04868(21) 9.1(1) min ß- to 236U; SF 1(-) N/A 1767.2 -
237Pa 237.05115(11) 8.7(2) min ß- to 237U (1/2+) N/A 1765.96 -
238Pa 238.05450(6) 2.27(9) min ß- to 238U; SF (3-)# N/A 1774.04 -
239Pa 239.05726(21)# 1.8(5) h ß- to 239U (3/2)(-#) N/A 1782.12 -
240Pa 240.06098(32)# 2# min ß- to 240U N/A N/A 1780.88 -
Protactinium Elemental Symbol

Recent Research & Development for Protactinium

  • Protactinium-231 and thorium-230 abundances and high scavenging rates in the western arctic ocean. Edmonds HN, Moran SB, Hoff JA, Smith JN, Edwards RL.. Science. 1998 Apr 17
  • Measurement of femtogram quantities of protactinium in silicate rock samples by multicollector inductively coupled plasma mass spectrometry.. Regelous M, Turner SP, Elliott TR, Rostami K, Hawkesworth CJ.. Anal Chem. 2004 Jul 1
  • Calculated optical properties of thorium, protactinium, and uranium metals.. Gasche T, Brooks MS, Johansson B.. Phys Rev B Condens Matter. 1996 Jul 15
  • Thermodynamic study of the complexation of protactinium(V) with diethylenetriaminepentaacetic acid.. Mendes M, Leguay S, Le Naour C, Hamadi S, Roques J, Moisy P, Guillaumont D, Topin S, Aupiais J, Den Auwer C, Hennig C.. Inorg Chem. 2013 Jul 1
  • Absorption of thorium and protactinium from the gastrointestinal tract in adult mice and rats and neonatal rats.. Sullivan MF, Miller BM, Ryan JL.. Health Phys. 1983 Apr
  • Melting of the Earth's lithospheric mantle inferred from protactinium-thorium-uranium isotopic data. Asmerom Y, Cheng H, Thomas R, Hirschmann M, Edwards RL.. Nature. 2000 Jul 20
  • Peculiar protactinium.. Wilson R.. Nat Chem. 2012 Jun 21
  • Relativistic small-core pseudopotentials for actinium, thorium, and protactinium.. Weigand A, Cao X, Hangele T, Dolg M.. J Phys Chem A. 2014 Apr 3
  • Protactinium-231 Dating of Carbonates by Thermal Ionization Mass Spectrometry: Implications for Quaternary Climate Change. Edwards RL, Cheng H, Murrell MT, Goldstein SJ.. Science. 1997 May 2
  • First structural characterization of a protactinium(V) single oxo bond in aqueous media.. Le Naour C, Trubert D, Di Giandomenico MV, Fillaux C, Den Auwer C, Moisy P, Hennig C.. Inorg Chem. 2005 Dec 12