Protactinium (Pa)

Protactinium (formerly: protoactinium) is a chemical element with the atomic number 91 in the periodic table. With an approximate abundance of a few parts per trillion, it’s one of the rarest naturally occurring elements in Earth’s crust. 

This member of the actinides family of periodic table elements is a poisonous and expensive chemical that emits high radioactivity. Due to its scarcity, protactinium has no practical use and it’s used exclusively for basic scientific research.  

Chemical and Physical Properties of Protactinium

Atomic number91
Atomic weight (mass)231.0359 g.mol-1
Group (number)Actinides
Period7 (f-block)
ColorA silvery-gray radioactive metal
Physical stateSolid at room temperature
Half-lifeFrom 3.8(+4.6−1.4) milliseconds to 3.276(11)×104 years
Electronegativity according to Pauling1.5
Density15.37 at 20°C
Melting point1572°C, 2862°F, 1845 K
Boiling point4000°C, 7232°F, 4273 K
Van der Waals radius160.6 pm
Ionic radius0.78Å
Most characteristic isotope231Pa to 240Pa
Electronic shell[Rn] 5f2 6d1 7s2
The energy of the first ionization568 kJ mol‑1
The energy of the second ionization1150 kJ mol‑1
Discovery dateIn 1913 by Kasimir Fajans and Otto Göhring (O.H. Gohring)

With the periodic table symbol Pa, atomic number 91, atomic mass of 231.0359 g.mol-1, and electron configuration [Rn] 5f2 6d1 7s2, protactinium is a dense, radioactive metal with a silvery-gray luster that is classified in the actinides family of the periodic table. It reaches its boiling point at 4000°C, 7232°F, 4273 K, while the melting point is achieved at 1572°C, 2862°F, 1845 K. 

Protactinium has an electronegativity of 1.5 according to Pauling, whereas the atomic radius according to van der Waals is 160.6 pm. When exposed to air, this shiny and malleable radioactive metal slowly tarnishes to form oxide. At temperatures below 1.4 K, protactinium displays the properties of a superconductor.

How Was Protactinium Discovered?

While conceptualizing the periodic system of elements, the great Russian chemist Dmitri Mendeleev predicted the existence of an element that was supposed to be classified between thorium and uranium. The new element was postulated to form the highest oxide X2O5, similarly to elements niobium and tantalum. In 1871, the actinide element group was still an unknown concept. 

The Contribution of Sir William Crookes

In 1900, the English chemist Sir William Crookes (1832-1919) observed a new radioactive substance in several different samples of uranium ores. He labeled this new chemical as uranium-X. Further analyzing his finding, Sir Crooks came to a conclusion that uranium-X is, in fact, composed of two different substances, distinguished as uranium-X1 (UX-1) and uranium-X2 (UX-2). 

The Discovery of Kasimir Fajans and Otto Göhring

While studying uranium’s decay chain in 1913, the Polish American physical chemist Kasimir Fajans (1887 – 1975) in collaboration with the German chemist Otto Göhring (1889 – 1915) managed to identify a new short-lived isotope in Karlsruhe, Germany. Since its half-life was extremely brief, they first labeled the element brevium. The half-life of the observed isotope protactinium-234m was 1.17 minutes. Considering the fact that this isotope was a product of the uranium radioactive decay series, the discoverers eventually decided that this form is the same as uranium-X2 – previously observed by Sir William Crooks. 

The Discovery of Otto Hahn and Lise Meitner

Several years later, two other groups of scientists succeeded in confirming the scientific evidence derived by Fajans and Göhring. In 1917, the German chemist Otto Hahn and the Austrian physicist Lise Meitner conducted research on a silica residue they had extracted from a pitchblende sample (uranium oxide) at the Kaiser Wilhelm Institute in Berlin. These two scientists were in a search of the ‘mother substance’ in which Fajans and Göhring detected the protactinium isotope as a decay product. 

While conducting their experiments, Hahn and Meitner also observed a form of the new element which turned out to be the most stable isotope of protactinium. It was the protactinium-231 form, with a half-life of 32. 670 years. In 1949, IUPAC acknowledged the work of these two scientists and credited Otto Hahn and Lise Meitner of Germany as the discoverers of the new radioactive metal.

The Contribution of Aristid von Grosse

The German nuclear chemist and Hahn’s colleague, Aristid von Grosse (1905-1985), isolated protactinium oxide (Pa2O5) in 1927. This is the highest and most stable oxide of element 91. Grosse achieved it by converting the protactinium isotope to the iodide (PaI5). This step of Grosse’s experiment was followed by the decomposition of the substance with a heated filament in a high vacuum. 

How Did Protactinium Get Its Name?

The name protactinium is derived by adding the Greek prefix ‘protos’ to the name of the chemical element ‘actinium’. Thus, the word ‘protactinium’ means “parent of actinium”, which indicates the fact that element actinium is a product of the radioactive decay of this new element

Where Can You Find Protactinium?

Protactinium can naturally be found in minute amounts in uranium ores. Its occurrence is calculated to be up to 3 parts per million. As the richest uranium source, the pitchblende ore from Zaire contains about one part Pa-231 in 10 million parts of ore. Trace amounts of element 91 also occur in soil, rock, surface water, groundwater – even in some animals and plants.

For commercial purposes, protactinium metal can also be obtained as a byproduct of the chemical elements uranium, plutonium, or thorium. It is also produced through nuclear fission or by extraction from spent nuclear fuel.

Protactinium in Everyday Life

Due to the fact that this chemical element is extremely scarce, highly radioactive, and toxic, it has no other practical uses apart from scientific research purposes.

How Dangerous Is Protactinium?

Being a highly radioactive and toxic substance, protactinium can be extremely harmful and pose great danger. Since this is one of the rarest chemicals that can be found, this claim is especially valid for the scientists who include it in their studies and research. 

Like all radioactive elements, protactinium is also classified as a carcinogenic substance. The ionizing radiation of the alpha particles (5.0 MeV) emitted by this chemical element is mostly accumulated in the bones, kidneys, and liver. Consumption of food or water contaminated with protactinium, as well as inhalation of its radioactive dust particles, presents the greatest risk of exposure. 

Environmental Effects of Protactinium

This highly radiotoxic metallic element is considered an extremely dangerous and hazardous substance for all life forms in the geological, biological, or aquatic systems of our environment. 

Isotopes of Protactinium

There are 30 isotopes of element 91, with atomic mass ranging from Pa-211 to Pa-240. Protactinium-231 is the most stable isotope of protactinium. It has a half-life of about 32.760 years and decays into actinium-227 through alpha decay. 

Protactinium-231 is an alpha emitter that is formed by the decay of uranium-235. On the other hand, the beta radiating protactinium-234 is formed as a result of uranium-238 decay.

All isotopes of this chemical element are radioactive. Most radioisotopes have an extremely short half-life – usually less than 1.8 seconds before they undergo an alpha decay or electron capture mode to form isotopes of Th, Ac, U, Ti, Ne, Pb, or Pa. Protactinium also has two nuclear isomers, 217mPa (with a half-life of 1.2 milliseconds) and 234mPa (with a half-life of 1.17 minutes). 


[n 1]



ZNIsotopic mass (Da)

[n 2][n 3]


[n 4]



[n 5]



[n 6]

Spin and


[n 7][n 4]

Natural abundance (mole fraction)
Excitation energyNormal proportionRange of variation
211Pa[2] 91120 3.8(+4.6−1.4) msα207Ac9/2−#  
212Pa 91121212.02320(8)8(5) ms

[5.1(+61−19) ms]

213Pa 91122213.02111(8)7(3) ms

[5.3(+40−16) ms]

214Pa 91123214.02092(8)17(3) msα210Ac   
215Pa 91124215.01919(9)14(2) msα211Ac9/2−#  
216Pa 91125216.01911(8)105(12) msα (80%)212Ac   
β+ (20%)216Th
217Pa 91126217.01832(6)3.48(9) msα213Ac9/2−#  
218Pa 91127218.020042(26)0.113(1) msα214Ac   
219Pa 91128219.01988(6)53(10) nsα215Ac9/2−  
β+ (5×10−9%)219Th
220Pa 91129220.02188(6)780(160) nsα216Ac1−#  
221Pa 91130221.02188(6)4.9(8) μsα217Ac9/2−  
222Pa 91131222.02374(8)#3.2(3) msα218Ac   
223Pa 91132223.02396(8)5.1(6) msα219Ac   
β+ (.001%)223Th
224Pa 91133224.025626(17)844(19) msα (99.9%)220Ac5−#  
β+ (.1%)224Th
225Pa 91134225.02613(8)1.7(2) sα221Ac5/2−#  
226Pa 91135226.027948(12)1.8(2) minα (74%)222Ac   
β+ (26%)226Th
227Pa 91136227.028805(8)38.3(3) minα (85%)223Ac(5/2−)  
EC (15%)227Th
228Pa 91137228.031051(5)22(1) hβ+ (98.15%)228Th3+  
α (1.85%)224Ac
229Pa 91138229.0320968(30)1.50(5) dEC (99.52%)229Th(5/2+)  
α (.48%)225Ac
230Pa 91139230.034541(4)17.4(5) dβ+ (91.6%)230Th(2−)  
β (8.4%)230U
α (.00319%)226Ac
231PaProtactinium91140231.0358840(24)3.276(11)×104 yα227Ac3/2−1.0000[n 8] 
CD (1.34×10−9%)207Tl


SF (3×10−10%)(various)
CD (10−12%)208Pb


232Pa 91141232.038592(8)1.31(2) dβ232U(2−)  
EC (.003%)232Th
233Pa 91142233.0402473(23)26.975(13) dβ233U3/2−Trace[n 9] 
234PaUranium Z91143234.043308(5)6.70(5) hβ234U4+Trace[n 10] 
SF (3×10−10%)(various)
235Pa 91144235.04544(5)24.44(11) minβ235U(3/2−)  
236Pa 91145236.04868(21)9.1(1) minβ236U1(−)  
β, SF (6×10−8%)(various)
237Pa 91146237.05115(11)8.7(2) minβ237U(1/2+)  
238Pa 91147238.05450(6)2.27(9) minβ238U(3−)#  
β, SF (2.6×10−6%)(various)
239Pa 91148239.05726(21)#1.8(5) hβ239U(3/2)(−#)  
240Pa 91149240.06098(32)#2# minβ240U   

Source: Wikipedia

List of Protactinium Compounds 

Protactinium is usually present in the oxidation states of +2, +3, +4, and +5 in the chemical compounds it forms. While it’s chemically non-reactive to alkalis, protactinium is rapidly attacked by oxygen, steam, and inorganic acids. 

When diluted, protactinium in the +5 state rapidly hydrolyzes by combining with hydroxide ions. In this way, it forms soluble or insoluble hydroxy-oxide solids which have a tendency to stick to the surfaces of vessels in which this chemical element is contained.

There are a number of protactinium chemical compounds, among which some are characterized with a distinct coloring. Some of them include:

  • Protactinium (V) chloride
  • Protactinium(IV) oxide
  • Protactinium(V) oxide
  • Protactinocene

5 Interesting Facts and Explanations

  1. According to the Los Alamos National Laboratory, protactinium is one of the rarest and expensive naturally occurring chemical elements. 
  2. In 1949, the International Union for Pure and Applied Chemistry (IUPAC) formalized the name protactinium for element 91.
  3. The scientists Fajans and Hahn also discovered the formula that defines the conditions of the precipitation and absorption of radioactive substances, which was of immense help to the scientists in the future discovery of the new radioactive elements.
  4. The same year Hahn and Meitner discovered protactinium, the British chemists Frederick Soddy and John Cranston also succeeded in discovering element 91 at the University of Glasgow, Scotland. 
  5. The chemists employed at the Chemists at Great Britain Atomic Energy Authority have succeeded in extracting 125 grams of 99.9% pure protactinium, which surpasses the total amount that has ever been isolated in the world since its discovery by 100 times! This was achieved by processing 60 tonnes of waste material in a 12-stage process, costing about $500,000.