Name: Protactinium
Symbol: Pa
Atomic Number: 91
Atomic Weight: 231.035880
Family: Rare Earth Elements
CAS RN: 7440-13-3
Description: A silvery metallic rare earth metal, highly toxic and radioactive.
State (25 C): Solid
Oxidation states:  +4, +5

Molar Volume: 15.03 cm3/mole
Valence Electrons: 5f26d17s2

Boiling Point: 4300K, 4027C, 7281F
Melting Point: 2113K, 1600C, 2912F

Electrons Energy Level: 2, 8, 18, 32, 20, 9, 2
Isotopes: 29 + None Stable = 2 Meta States
Heat of Vaporization: 481.2 kJ/mol
Heat of Fusion: 12.3 kJ/mol
Density: 15.37 g/cm3 @ 300K
Specific Heat: 0.12 J/gK
Atomic Radius: 160.6 pm
Ionic Radius: 0.78
Electronegativity: 1.5 (Pauling),  1.014 (Allrod Rochow)

1s2 2s2p6 3s2p6d10 4s2p6d10f14 5s2p6d10f2 6s2p6d1 7s2


The existence of an element between thorium and uranium was predicted to exist by Mendeleev in 1871.  In 1900 William crookes isolated protactinium as a radioactive material from uranium which he could not identify.

Protactinium was first identified in 1913, when Kasimir Fajans and O.H. Gohring encountered short-lived isotope 234m-Pa, with a half-life of about 1.17 minutes, during their studies of the decay chain of 238-U.  They gave the new element the name Brevium (Latin brevis, brief, short); because of the short life-time of the transition between Th-234 and U-234, a longer-lived isotope was eventually isolated and the name was changed to Protoactinium in 1918 when two groups of scientists (Otto Hahn and Lise Meitner of Germany and Frederick Soddy and John Cranston of the UK) independently discovered 231-Pa, and shortened to Protactinium in 1949.

Aristid V. Grosse prepared 2 mg of Pa2O5 in 1927, and later on managed to isolate Protactinium for the first time in 1934 from 0.1 mg of Pa2O5, first converting the oxide to an iodide and then cracking it in a high vacuum by an electrically heated filament by the reaction:

2PaI5 rarrow.gif (63 bytes) 2Pa + 5I2

In 1961, the United Kingdom Atomic Energy Authority was able to produce 125 g of 99.9% pure protactinium, processing 60 tons of waste material in a 12-stage process and spending 500,000 USD; this was the world's only supply of the element for many years to come, and it is reported that the metal was sold to laboratories for a cost of 2,800 USD / g in the following years.

About 60 tons of pitchblende ore (which contains uranium, radium, and a host of other radioactive elements) yields about 125 grams of protactinium.


It is malleable, shiny, silver-gray, radioactive.  It does not tarnish rapidly in air, it is attacked by oxygen, steam and acids, but not by alkalis.  Protactinium belongs to the actinide group, with a bright metallic luster that it retains for some time in the air.  It is superconductive at temperatures below 1.4oK.  The metal is extrememly rare, very radioactive and highly poisonous.

2s2 2p6
3s2 3p6 3d10
4s2 4p6 4d10 4f14
5s2 5p6 5d10 5f2
6s2 6p6 6d1


Protactinium occurs in pitchblende to the extent of about 1 part  231Pa to 10 million of ore.  Some ores from the Democratic Republic of the Congo have about 3 ppm.


Due to its scarcity, high radioactivity and toxicity, there are currently no uses for protactinium outside of basic scientific research.  Protactinium is one of the rarest and most expensive naturally occurring elements.

Protactinium-231, which is formed by the alpha decay of Uranium-235, could possibly sustain a nuclear chain reaction and might, in principle, be used to build a nuclear weapon.  The critical mass, according to Walter Seifritz, is 750180 kg.  Other authors conclude that no chain reactions are possible in Protactinium-231.


A number of protactinium compounds are known, some of which are colored.

Fluorides Chlorides Bromides
PaF4 PaCl4 PaBr4
PaF5 PaCl5 PaBr5
Iodides Oxides
PaI3 PaO
PaI4 PaO2
PaI5 Pa2O5


29 radioistopes of protactinium have been characterized, with the most stable being 231-Pa with a half-life of 32760 years, 233-Pa with a half-life of 26.967 days, and 230-Pa with a half-life of 17.4 days.  All of the remaining radioactive isotopes have half-lifes that are less than 1.6 days, and the majority of these have half lifes that are less than 1.8 seconds.  This element also has 2 meta states, 217m-Pa (t 1.15 milliseconds) and 234m-Pa (t 1.17 minutes).

The primary decay mode before the most stable isotope, 231-Pa, is alpha decay and the primary mode after is beta minus decay.  The primary decay products before 231-Pa are element Ac (actinium) isotopes and the primary products after are element U (uranium) isotopes.

The element is an alpha emitter (5.0 MeV) and is a radiological hazard similar to polonium.

atom.gif (700 bytes)

Atomic Mass
212Pa 212.02320 8 ms
213Pa 213.02111 7 ms
214Pa 214.02092 17 ms
215Pa 215.01919 14 ms
216Pa 216.01911 105 ms
217Pa 217.01832 3.48 ms
217mPa   1.08 ms
218Pa 218.020042 0.113 ms
219Pa 219.01988 53 ns
220Pa 220.02188 780 ns
221Pa 221.02188 4.9 s
222Pa 222.02374 3.2 ms
223Pa 223.02396 5.1 ms
224Pa 224.025626 844 ms
225Pa 225.02613 1.7 seconds
226Pa 226.027948 1.8 minutes
227Pa 227.028805 38.3 minutes
228Pa 228.031051 22 hours
229Pa 229.0320968 1.50 days
230Pa 230.034541 17.4 days
231Pa 231.0358840 3.276 x 104 years
232Pa 232.038592 1.31 days
233Pa 233.0402473 26.975 days
234Pa 234.043308 6.70 hours
234mPa   1.17 minutes
235Pa 235.04544 24.44 minutes
236Pa 236.04868 9.1 minutes
237Pa 237.05115 8.7 minutes
238Pa 238.05450 2.27 minutes
239Pa 239.05726 1.8 hours
240Pa 240.06098 ~2 minutes


40px-Skull_and_crossbones.svg.jpg (1420 bytes) Protactinium is both toxic and highly radioactive.  It requires precautions similar to those used when handling plutonium.

The major health concern is cancer resulting from the ionizing radiation emitted by protactinium deposited in the skeleton, liver, and kidneys. The health risks associated with protactinium-234m are included with those for uranium-238. Protactinium-234m decays by emitting an energetic beta particle so precautions against this radiation are needed when handling uranium; for example, heavy rubber gloves are worn to protect the hands and forearms.

The inhalation risk factor for protactinium-231 represents one of the largest risk factors for any radionuclide. Actinium-227 and its decay products account for more than 80% of this inhalation risk. While the risk factor for ingestion is much lower than for inhalation, ingestion is generally the most common means of entry into the body.

Similar to other radionuclides, the risk coefficient for tap water is about 75% of that shown for dietary ingestion.

In addition to risks from internal exposures, there is a risk from external gamma exposure to protactinium-231.

atom.gif (700 bytes)

Protactinium Data

Atomic Structure

  • Atomic Radius: 160.6 pm
  • Atomic Volume: 15cm3/mol
  • Covalent Radius:
  • Cross Section (Thermal Neutron Capture) Barns: 200.6
  • Crystal Structure: Orthorhombic
  • Electron Configuration:
    1s2 2s2p6 3s2p6d10 4s2p6d10f14 5s2p6d10f2 6s2p6d1 7s2
  • Electrons per Energy Level: 2, 8, 18, 32, 20, 9, 2
  • Ionic Radius: 0.78
  • Filling Orbital: 5f2
  • Number of Electrons (with no charge): 91
  • Number of Neutrons (most common/stable nuclide): 122
  • Number of Protons: 91
  • Oxidation States: 5, 4
  • Valence Electrons: 5f2 6d1 7s2

Chemical Properties

  • Electrochemical Equivalent: 1.724 g/amp-hr
  • Electron Work Function:
  • Electronegativity: .5 (Pauling); 1.014 (Allrod Rochow)
  • Heat of Fusion: 12.3 kJ/mol
  • Incompatibilities:
  • Ionization Potential
    • First: 5.89
  • Valence Electron Potential (-eV): 92

Physical Properties

  • Atomic Mass Average: 213.0359
  • Boiling Point: 4300K, 4027C, 7281F
  • Coefficient of Lineal Thermal Expansion/K-1: 7.3E-6
  • Conductivity
    Electrical: 0.0529 106/cm
    Thermal: 0.47 W/cmK
  • Density: 15.37 g/cm3 @ 300K
  • Description:
    Radioactive rare earth metal found naturally in uranium ores.
  • Enthalpy of Fusion: 12.34 kJ/mole
  • Flammablity Class:
  • Freezing Point: see melting point
  • Heat of Vaporization: 481.2 kJ/mol
  • Melting Point: 2113K, 1600C, 2912F
  • Molar Volume: 15.03 cm3/mole
  • Physical State (at 20C & 1atm): Solid
  • Specific Heat: 0.12 J/gK

Regulatory / Health

  • CAS Number
    • 7440-13-3
  • NFPA 704
    • Health:
    • Fire:
    • Reactivity:
    • Special Hazard: Radioactive<
    • OSHA Permissible Exposure Limit (PEL)
      • No limits set by OSHA
    • OSHA PEL Vacated 1989
      • No limits set by OSHA
    • NIOSH Recommended Exposure Limit (REL)
      • No limits set by NIOSH
    • Levels In Humans:
      Note: this data represents naturally occuring levels of elements in the typical human, it DOES NOT represent recommended daily allowances.
      • Blood/mg dm-3: nil
      • Bone/p.p.m: nil
      • Liver/p.p.m: nil
      • Muscle/p.p.m: nil
      • Daily Dietary Intake: nil
      • Total Mass In Avg. 70kg human: nil

    Who / Where / When / How

    • Discoverer: Kasimir Fajans and O.H. Ghring
    • Discovery Location: Karlsruhe Germany
    • Discovery Year: 1913
    • Name Origin:
      Greek: protos (first); its is the parent of actinium, which is formed by radioactive decay.
    • Abundance:
      • Estimated Crustal: 1.410-6 milligrams per kilogram
      • Estimated Oceanic: 510-11 milligrams per liter
      • Atmosphere/p.p.m.: N/A
      • Sun (Relative to H=1E12): N/A
    • Sources:
      Occurs in minute trace amounts in uranium ores such as pitchblende. Found among fission products of uranium, thorium, and plutonium. In 1961 the UK Atomic Energy Authority produced 125 g of pure protactinium from uranium fuel elements, which represents the major world stock of pratactinium.
    • Uses:
    • Additional Notes:
      Different sources credit the discovery of Protactinium because Kasimir Fajans and O.H. Ghring discovered Pa-231 in 1913, while Lise Meitner and Otto Hahn (Germany), K. Fajans (Germany) and F. Soddy, J.A. Cranston and A. Fleck (Scotland) discovered Pa-231 in 1917. We suspect that geo-political rivalries surrounding the First World War resulted in the 1913 discovery being omitted from many sources.

*All Data Varies Depending on Source


Lattice Constant (): 3.920