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What is Protactinium

Protactinium is a highly radioactive metal belonging to the family of actinides and denoted by the chemical symbol Pa [1]. It has 27 isotopes with mass numbers ranging from 212 to 238 out of which protactinium-231 is the most stable one with a half-life of 3.276 x 104 years. After which, it forms actinium-277 through alpha decay [3].

Protactinium Symbol


Origin of its Name: Its name is derived from the Greek word ‘protos’, meaning first, added as a prefix to the element actinium [1].

Who Discovered Protactinium: Kasimir Fajans and Otto Gohring were the discoverers of the element [1].

When and How was it Discovered

In 1900, William Crookes isolated an unidentified radioactive metal from uranium. However, it was only in 1913 when Fajans and Gohring showed that the new element underwent beta-decay and confirmed it to fall in the sequence of elements through uranium decay. It was identified to be isotope protactinium-234 with a half-life of 6 hours and 42 minutes [1].

In 1918, Lise Meitner at the Kaiser Wilhelm Institute, Berlin extracted another isotope protactinium-231 from uranium ore pitchblende [1].


Protactinium Identification

Atomic number

91 [1]

CAS number

7440-13-3 [1]

Position in the periodic table [1]







Location of Protactinium in the Periodic Table

Properties and Characteristics of Protactinium

General Properties

Relative atomic mass 231.036 [1]
Atomic mass 231.036 atomic mass units [5]

Physical Properties

Color/appearance Silvery [1]
Luster Metallic [6]
Melting point/freezing point 1572°C (2862°F) [1]
Boiling point 4000°C (7232°F) [1]
Density 15.4 g/cm3 [1]
State at standard room temperature (solid/liquid/gas) Solid [1]
Hardness (Brinell, Mohs, Vickers) Unknown [8]
Thermal conductivity 47 W m-1K-1
Electrical conductivity 5.6 x 106 Sm-1

Chemical Properties

Oxidation state/Oxidation number (+2), +3, +4, +5 [1]

Atomic Data of Protactinium (Element 91)

Electron configuration (noble gas configuration) [Rn] 5f26d17s2 [1]
Crystal Structure Face-centered cubic (fcc) [3]
Atomic structure [5]
–          Number of Electrons 91
–          Number of Neutrons 140
–          Number of Protons 91
Energy levels [9]
–          First energy level 2
–          Second energy level 8
–          Third energy level 18
–          Fourth energy level 32
–          Fifth energy level 20
–          Sixth energy level 9
–          Seventh energy level 2
Radius of atom
       –       Atomic Radius 2.43 Å [1]
       –       Covalent Radius 1.84 Å [1]
Electronegativity 1.5 [7]
Ionization energy [1]


1st 2nd 3rd 4th 5th 6th 7th

Protactinium Atomic Structure (Bohr Model)

Uses of Protactinium

There are no practical applications of the element outside basic scientific research [1].

Protactinium Hazards

Although the radioactive nature of the element makes it highly toxic, its scarcity in nature does not pose any serious threats [10].

Interesting Facts

  • In 1961, 125 g of pure Pa was produced at the Great Britain Atomic Energy Authority by processing 55,000 kg of ore that led to an expenditure of about $500,000 [2].
  • Minute amounts of the element exist in uranium ores as well as found in spent fuel rods obtained from nuclear reactors [1].

Protactinium Cost

The price of the metal may vary between $250 to $300 per gram.



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