Protactinium Facts – Symbol Pa or Atomic Number 91

Protactinium is a dense, silvery-gray metallic element with the symbol Pa and atomic number 91. It is a radioactive actinide found in trace amounts in uranium ores. Protactinium is one of the rarest naturally occurring elements in Earth’s crust and has limited commercial applications due to its scarcity, toxicity, and intense radioactivity. However, it has important scientific uses in nuclear chemistry and isotope geology. Protactinium exists in several oxidation states but is most stable in the +5 state. It is highly reactive with oxygen, halogens, and acids, forming a variety of complex compounds. Here are protactinium facts, including it properties, uses, sources, and health effect.

Key Takeaways: Protactinium Facts

• Protactinium (Pa) is a rare, radioactive metal with atomic number 91 and a silvery-gray appearance.
• It belongs to the actinide series and occurs naturally in trace amounts in uranium ores as part of the uranium-235 decay chain.
• The element was independently discovered in 1917–1918 by Hahn and Meitner, among others, and named for its position “before actinium” in the decay series.
• Protactinium’s most stable isotope, Pa-231, has a half-life of 32,760 years and is used in radiometric dating of ocean sediments.
• It is toxic and highly radioactive, with no biological role and limited use outside of nuclear science research.
• Protactinium exhibits multiple oxidation states, especially +5, and forms oxides and halide compounds.
• Due to its scarcity, difficulty of extraction, and radiological hazards, it is extremely expensive, costing thousands of dollars per gram.
• The element is produced in small quantities by a few countries with advanced nuclear capabilities.

History of Discovery, Isolation, and Naming

The discovery of protactinium involved multiple scientists and iterations:

• 1898: German chemists Kasper Fajans and Oswald Helmuth Göhring identified a short-lived isotope, which they named brevium (Pa-234), due to its short half-life.
• 1917–1918: British scientists Frederick Soddy and John Cranston, and German chemists Otto Hahn and Lise Meitner, independently discovered the longer-lived isotope Pa-231, which is the naturally occurring isotope. Hahn and Meitner are generally credited with the discovery of the element.
• The name “protoactinium” was proposed independently in 1917–1918 by Otto Hahn and Lise Meitner (Germany) and Frederick Soddy and John Cranston (UK), based on the element’s role as a precursor to actinium in the uranium-235 decay chain.
  In 1949, IUPAC officially shortened the name to protactinium.
• Name origin: The name “protactinium” derives from the Greek “protos,” meaning “before,” and “actinium,” as it decays into actinium-227.

In 1949, IUPAC officially shortened the element name from “protoactinium” to “protactinium.”

The first isolation of pure metallic protactinium was achieved in 1961 by the UK Atomic Energy Authority using a complex ion-exchange process involving tons of uranium ore.

Periodic Table Location and Group

Protactinium is part of the actinide series, which comprises 15 elements from actinium (89) to lawrencium (103). Its location on the periodic table is between thorium and uranium. Like other actinides, it displays a range of oxidation states and complex electron behavior due to the involvement of 5f orbitals.

• Symbol: Pa
• Atomic Number: 91
• Period: 7
• Block: f-block
• Group: Actinides (no official group number)

Appearance and Characteristics

Protactinium is a dense, hard, and ductile metal with a bright, silvery-gray appearance when freshly cut. It is lighter than uranium but denser and more rigid than thorium. In general, its properties are intermediate between these two elements. It oxidizes in air, forming a dull coating. It is superconducting below 1.4 K and has a high melting point, reflecting strong metallic bonding. The metal crystallizes in the body-centered tetragonal system, but transitions to a face-centered cubic allotrope upon cooling from a temperature of around 1200 °C.

Key characteristics:

• Highly radioactive
• Chemically reactive
• Paramagnetic
• Toxic
• Dense (15.37 g/cm³)
• Difficult to handle and store

Isotopes

Protactinium has no stable isotopes. The most significant isotopes are:

In total, over 30 isotopes of protactinium are known, ranging from mass numbers 212 to 238, with all being radioactive.

Origin, Abundance, and Sources

Protactinium is a primordial element, formed by nucleosynthesis in stars. All of the original protactinium decayed long ago. On Earth, it occurs naturally in uranium ores such as pitchblende and carnotite.

Abundance:

• Earth’s crust: ~0.1 ppb
• Oceans: negligible
• Found at about 1 part per trillion in uranium ores, although it reaches 3 ppt in some ores

Natural source:

• Most protactinium-231 is produced via α decay of uranium-235.
• Pa-234 is a short-lived intermediate in the decay chain of uranium-238.

Production methods:

• Extracted as a byproduct of uranium processing.
• Produced synthetically via neutron irradiation of thorium-232 or uranium-238.

Uses of Protactinium

Due to its rarity, high radioactivity, and toxicity, protactinium has limited practical applications:

Scientific and Industrial Uses:

• Tracer in geology: Pa-231 is used in radiometric dating, particularly for deep-sea sediments (Pa/Th dating).
• Nuclear science: Important in understanding actinide chemistry and decay chains.
• Target material: Occasionally used in research reactors for producing isotopes.

Historical use:

• The UK and US conducted extensive studies during nuclear weapons development and early nuclear reactor programs, but protactinium’s cost and hazards limited its deployment.

Oxidation States

Protactinium exhibits oxidation states from +2 to +5, with the most common being:

• +5 (most stable and common in aqueous chemistry)
• +4 (forms some halides and oxides)
• +3, +2 (occurs under highly reducing conditions)

Chemistry and Compounds

Protactinium chemistry resembles that of uranium and thorium but is more limited due to its scarcity. It forms both ionic and covalent compounds.

Common compounds:

• Pa₂O₅ (protactinium pentoxide): A white solid; major oxidation state +5.
• PaCl₅ and PaF₅: Volatile halides used in early purification methods.
• PaBr₄ and PaI₅: Less common halides.
• Fluorocomplexes: PaF₇²⁻ and PaF₈³⁻ in hydrofluoric acid.

It forms coordination complexes with oxygen-donor ligands, and its aqueous ions strongly hydrolyze.

Biological Role, Health Effects, and Toxicity

Biological Role:

Protactinium has no known biological role in humans or other organisms.

Health Effects:

• Highly toxic due to intense radioactivity.
• Inhalation or ingestion damages tissues and increases cancer risk.
• Accumulates in bones, liver, and kidneys, similar to other actinides. However, only around 0.05% of ingested or inhaled protactinium gets absorbed, with the remainder being excreted.

Environmental impact:

• Usually immobile in the environment due to strong binding to soils and sediments.
• Biological uptake is minimal, but decay products pose hazards.

Table of Key Protactinium Facts for Scientists

FAQs About Protactinium

What is protactinium used for?

Protactinium has very limited practical applications due to its rarity, cost, and radioactivity. However, it is used in:

• Scientific research on nuclear chemistry and actinide behavior.
• Radiometric dating, especially Pa-231 in combination with Th-230 to study ocean sediments and climate history.
• Nuclear fuel cycle studies and as a theoretical intermediate in breeding fissile uranium-233 from thorium-232.

How much does protactinium cost?

Protactinium is extremely expensive, primarily because it is rare and difficult to isolate. The cost is approximately:

• $2800 per gram (estimated)
• In some cases, more than $1 million per kilogram

Prices vary depending on isotope purity and production method. Only a few grams are produced annually worldwide.

Is protactinium toxic?

Yes. Protactinium is highly toxic due to its radioactivity:

• Emits alpha particles that cause internal tissue damage if inhaled or ingested.
• Can accumulate in bones, kidneys, and liver.
• Long-term exposure increases cancer risk.
  Handling requires specialized facilities with shielding and ventilation.

Which countries produce protactinium?

Protactinium is produced in trace amounts in only a few countries with advanced nuclear capabilities:

• Russia (historically produced larger amounts)
• United Kingdom
• United States
• France
• India (limited research use)

Typical extraction is as a byproduct of uranium fuel processing or via synthesis in nuclear reactors.

What color is protactinium?

Metallic protactinium is silvery-gray with a shiny appearance when freshly prepared. It eventually tarnishes in air, forming a dull oxide layer.

Does protactinium glow in the dark?

No, protactinium does not visibly glow in the dark. While it is radioactive, it primarily emits alpha particles, which do not produce visible light. Unlike some radioactive materials (such as radium or tritium mixed with phosphors), protactinium does not undergo radioluminescence on its own. Glow only occurs under special conditions, such as embedding in a phosphorescent matrix or viewing in a vacuum with a sensitive detector.

Is protactinium a metal?

Yes. Protactinium is a metal, specifically a radioactive actinide metal. It is:

• Dense and malleable
• Conductive of heat and electricity
• Chemically reactive, especially with halogens and oxygen

What is the half-life of protactinium?

The most relevant isotope is protactinium-231, which has a half-life of 32,760 years. It is the only naturally occurring isotope and plays a role in geological dating.

Another isotope, Pa-234, has a much shorter half-life of 6.70 hours and occurs in the uranium-238 decay chain.

Is protactinium naturally occurring?

Yes. Protactinium-231 is naturally occurring, though extremely rare. It forms in the uranium-235 decay chain and is found in trace amounts in uranium ores.

Is protactinium radioactive?

Yes. All isotopes of protactinium are radioactive. Most decay by alpha or beta emission and produce decay products such as actinium or thorium.

How was protactinium discovered?

Protactinium was discovered in 1913–1918 by multiple researchers, including Otto Hahn, Lise Meitner, and Frederick Soddy. The element’s name reflects its role as a precursor to actinium in radioactive decay.

Can you buy protactinium?

Not easily. Due to its radioactivity, toxicity, and cost, protactinium is not commercially available to the public. It is tightly regulated and used only in authorized laboratories.

Does protactinium play a role in biology?

No. Protactinium has no known biological function and is not used in medicine due to its high radiotoxicity.

Common Misconceptions About Protactinium

1. Protactinium is just a synthetic element

False.
While many actinides are synthetic, protactinium-231 occurs naturally in trace amounts as part of the uranium-235 decay series. It is one of the few naturally occurring radioactive elements beyond uranium.

2. Protactinium is highly abundant in nature

False.
Protactinium is one of the rarest naturally occurring elements. Its concentration in the Earth’s crust is roughly 0.1 parts per billion. It is so scarce that early researchers had to process tons of uranium ore to isolate a few grams.

3. It has practical industrial uses

False.
Although once investigated for nuclear fuel applications, protactinium is not used industrially due to its cost, scarcity, and high radiotoxicity. Its use is largely restricted to scientific research.

4. Protactinium is safe to handle like other metals

False.
Protactinium is highly radioactive and must be handled only in specialized facilities with strict safety protocols. Direct contact or inhalation of dust can cause serious health effects due to internal alpha radiation.

5. All protactinium isotopes are short-lived

False.
While many protactinium isotopes are short-lived, Pa-231 has a half-life of 32,760 years. This makes it important in long-term geologic dating and environmental studies.

6. It’s no longer relevant to science

False.
Protactinium continues to be relevant in nuclear chemistry, radiometric dating, and actinide research, especially in understanding the behavior of radioactive decay series and deep-sea sedimentation.

7. Protactinium decays into uranium

False.
It’s the other way around. Protactinium-231 decays into actinium-227, not uranium. In fact, uranium-235 decays into protactinium as part of its decay series.

References

• Emsley, John (2011). Nature’s building blocks: An A-Z Guide to the Elements. Oxford University Press. ISBN 978-0-19-960563-7.
• Fajans, K.; Gohring, O. (1913). “Über die komplexe Natur des Ur X”. Naturwissenschaften. 1 (14): 339. doi:10.1007/BF01495360
• Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
• Hammond, C. R. (2004). The Elements, in Handbook of Chemistry and Physics (81st ed.). CRC press. ISBN 978-0-8493-0485-9.
• Meitner, Lise (1918). “Die Muttersubstanz des Actiniums, Ein Neues Radioaktives Element von Langer Lebensdauer”. Zeitschrift für Elektrochemie und angewandte physikalische Chemie. 24 (11–12): 169–173. doi:10.1002/bbpc.19180241107