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What is Protactinium?

Protactinium (which was formerly protoactinium) is defined as a chemical element having the symbol Pa and the atomic number 91. It is a metal of silvery-grey colour, dense actinide that readily reacts with water vapour, oxygen, and also inorganic acids. It produces different chemical compounds in which protactinium is usually present in the oxidation state of +5, but it can also assume with +4 and even +3 or +2. Protactinium concentrations present in the crust of Earth are typically a few parts per trillion but can reach up to some parts per million in a few uraninite ore deposits.

Properties of Protactinium

Let us look at the important protactinium properties given as follows:

Physical Properties of Protactinium



Atomic Mass

231.0359 g.mol-1

Atomic Number


Discovered by

O.H. Gohring and K. Kajans in 1913

Chemical Properties of Protactinium

The chemical properties of Protactinium can be listed as follows:

  • The group of the Protactinium is Actinides

  • Its period is 7

  • Its block is f

  • It has an atomic number of 91

  • The state of protactinium at 20 ⁰C is Solid

  • Its electronic configuration is [Rn] 5f26d17s2

  • It has the ChemSpider ID as 22387

  • Protactinium melting point is 1572°C, 2862°F, 1845 K

  • Its boiling point is 4000°C, 7232°F, 4273 K

  • Its Density (g cm⁻³) is 15.4

  • The Key isotopes are 231Pa

  • Its Relative atomic mass is 231.036

  • Its CAS number is given as 7440-13-3

Preparation of Protactinium

Protactinium was separated for scientific experiments from uranium ores before the nuclear reactor’s advent. Nowadays, it is produced mostly as an intermediate product of nuclear fission in thorium high-temperature reactors:

\[_{90}^{232}\textrm{Th}\] + \[_{0}^{1}\textrm{n}\] \[\rightarrow\]  \[_{90}^{233}\textrm{Th}\]  \[\xrightarrow[22.3 min]{\beta^{-}}\] \[_{91}^{233}\textrm{Pa}\] \[\xrightarrow[26.967 d]{\beta^{-}}\] \[_{92}^{233}\textrm{U}\] 

The 231 isotopes are prepared by radiating thorium-230 with slow neutrons, converting to the beta-decaying thorium-231, or by irradiating thorium-232 with fast neutrons, generating two neutrons and thorium-231.

Occurrence of Protactinium

Protactinium is described as one of the most expensive and rarest naturally occurring elements. It can be found in the form of two isotopes – 234Pa and 231Pa, with the isotope 234Pa occurring in two different states of energy. Approximately all the natural protactinium is protactinium-231. It is an alpha emitter, and it can be formed by the uranium-235 decay, whereas the beta radiating protactinium-234 can be produced as a result of uranium-238 decay. Nearly all uranium-238 (nearly 99.8%) decays first to the shorter-lived 234mPa isomer.

Protactinium takes place in uraninite (pitchblende) at concentrations of nearly 0.3-3 parts 231Pa per million parts (ppm) of ore. At the same time, the usual content is closer to 0.3 ppm (for example, in Jáchymov, Czech Republic, a few ores from the Democratic Republic of the Congo have about three ppm. Protactinium is defined as homogeneously dispersed in most of the natural materials and in water, but at much lower concentrations in the order of one part per trillion, which corresponds to the radioactivity of 0.1 picocuries (pCi)/grams. There is up to 500 times more protactinium in sandy soil particles than in water, including in identical soil samples. Higher ratios of 2,000 and above are measured in clays and loam soils, such as bentonite.

Properties of Protactinium 

  • The protactinium metal is a radioactive, silvery, and shiny metal that slowly degrades in the presence of air to produce oxides. This metal contains up to 5 isotopes with their mass numbers ranging between 212 to 238, and protactinium 231 can be considered to be the most stable isotope containing a half-life of about 32,760 years. This isotope is formed by the decay of the element uranium 235 by emitting gamma radiation.

  • Protactinium is well known to be one of the rarest and most expensive naturally occurring elements on Earth. This metal also occurs about three parts per trillion and at times in parts per million in uranium ores. And, the uranium processing obtains it as a by-product. This element was discovered by two persons named O.H. Gohring and K.Kajans in 1913.

  • Protactinium is in a dense form as metal, and it is a silvery-grey having a bright metallic look. At temperatures below 1.4K, it is superconductive in nature. It reacts with inorganic acids, water vapour, and oxygen, producing its compounds. Also, a few of its compounds are coloured. In the state of the solid compound, protactinium is said to be the most stable in its oxidation state of +5. At the same time, it exists in the oxidation state of +4, even +3, and +2. The oxidation state of +5 rapidly gets hydrolyzed by combining with the hydroxide ions and forming both soluble and insoluble hydroxy-oxide solids in the solution state.

Protactinium Facts

Let us look at some of the protactinium facts.

  • The leading causes of exposure to this element in the human body are the intake of water and food and the inhalation of contaminated dust that consists of protactinium.

  • When ingested into the body, protactinium is considered to be toxic. It also affects the lungs and the gastrointestinal tract.

FAQ (Frequently Asked Questions)

Q1. List Some Applications of Protactinium?

Answer: While protactinium occurs in the periodic table between uranium and thorium elements, all of which include various protactinium uses, there is currently little need for protactinium outside of science testing due to its high radioactivity, high toxicity, and scarcity.

Protactinium-231 arises from the uranium-235 decay, which is produced in the nuclear reactors and by the reaction ²³²Th + n → ²³¹Th + 2n and the subsequent beta decay. When it was thought to be able to support a nuclear chain reaction, it could be used in principle to build nuclear weapons such as the physicist Walter Seifritz.

Q2. Give the Precautions of Protactinium?

Answer: As protactinium exists in fewer amounts in most natural materials and products, it is ingested with either water or food and inhaled with air. Only up to 0.05 percent of ingested protactinium can be absorbed into the blood, and the remainder is excreted. About 40% of the protactinium from the bones' blood deposits, about 2% to the kidneys, 15% goes to the liver, and the rest leaves the body.

Q3. What is the Protactinium Element?

Answer: Protactinium is a chemical element with atomic number 91 and the protactinium symbol Pa, which is a silvery-grey, dense, actinide metal that readily reacts with water vapour, inorganic acids, and oxygen.

Q4. Explain What Type of Element Protactinium is?

Answer: It is a radioactive and lanthanide metal having an orthorhombic crystal shape.