Pa · Element 91 · Actinide
One of the rarest and most expensive natural elements — extracting 125 g once cost half a million dollars.
Structure
Not a diagram of dots on rings — a Monte-Carlo sample of the actual probability density |ψ|² for each occupied subshell. Drag to rotate. Blue and violet mark opposite signs of the wavefunction, which is what makes bonding possible.
Measured values
Every bar shows where protactinium sits among all 118 elements for that property.
Sources: IUPAC 2021 standard atomic weights · CRC Handbook of Chemistry and Physics · NIST. Values marked ~ are predicted rather than measured.
Same element, different neutrons
All protactinium atoms have the same number of protons, 91, and differ only in their number of neutrons: those variants are its isotopes. The most common, Pa-231, has 140 neutrons and makes up 100% of natural protactinium.
| ISOTOPE | Neutrons | Natural Abundance | Half-life | Decay Mode |
|---|---|---|---|---|
| Pa-231 most common | 140 | 100% | 32570 yrs | α |
Here the classroom shortcut, rounded atomic mass (231.036) minus atomic number, gives the same answer: 140.
Decay modes
Data: IAEA Nuclear Data Section, LiveChart of Nuclides, ground-state evaluations, retrieved October 10, 2026. Abundance is the share of atoms in the natural element; a dash means the isotope has no standard natural abundance.
Size, to scale
Radius 163 pm — that is 0.163 nm, so about 3.07 million of them side by side would span a millimetre.
Thermal range
Protactinium is liquid over a 2455 K window, from 1845 K to 4300 K.
The story
One of the rarest and most expensive natural elements — extracting 125 g once cost half a million dollars.
Protactinium has a complex discovery history with multiple competing claims. The first evidence was found in 1913 by Kazimierz Fajans and Oswald Helmuth Göhring, German researchers who identified a short-lived isotope (Pa-234m) and named it "brevium" because of its brief half-life of just over one minute.
The longer-lived and more significant isotope protactinium-231 was identified in 1918 by British scientists Frederick Soddy and John Cranston, working independently from Otto Hahn and Lise Meitner in Germany. Both groups were investigating the uranium decay series and identified the same long-lived isotope.
The first isolation of a protactinium compound was achieved in 1927 by Aristid von Grosse (the metal followed in 1934), who obtained 2 milligrams of Pa₂O₅ from approximately 2 tons of pitchblende residue. This represented the culmination of years of painstaking chemical separation work involving thousands of fractional crystallizations.
The element's name changed from "brevium" to "protactinium" (meaning "before actinium") when the longer-lived Pa-231 isotope was characterized. This name reflects its position in the uranium-235 decay chain, where it decays to form actinium-227, hence being the "parent" of actinium.
The discovery exemplified both international collaboration and competition in early 20th-century radiochemistry. Despite working during World War I, scientists from different countries shared information and built upon each other's work, demonstrating the universal nature of scientific inquiry even during times of conflict.
Isolating protactinium required developing new radiochemical techniques for handling extremely small quantities of highly radioactive materials. The methods developed for protactinium separation became fundamental techniques used throughout nuclear chemistry and continue to influence modern radiochemical practices.
Applications
Protactinium serves primarily as a research tool in nuclear physics and radiochemistry laboratories. Scientists use protactinium isotopes to study nuclear decay processes, alpha emission mechanisms, and the fundamental properties of actinide elements. Its position between thorium and uranium makes it crucial for understanding the transition from naturally occurring to artificial transuranium elements.
The most significant practical application of protactinium is in uranium-protactinium dating methods for geological samples. The uranium-235 to protactinium-231 decay chain provides a reliable method for dating marine sediments and determining the age of geological formations spanning hundreds of thousands of years, particularly useful for paleoclimatology research.
Protactinium isotopes are valuable for investigating alpha decay systematics and nuclear structure theories. Researchers use protactinium-233 and other isotopes to test models of nuclear stability, study the quantum mechanical tunneling effect in alpha decay, and understand the nuclear properties that influence radioactive decay rates.
In analytical chemistry, protactinium compounds serve as tracers and reference standards for studying the chemical behavior of actinide elements. The element's unique chemical properties help researchers develop separation techniques for other radioactive elements and understand actinide chemistry in various chemical environments.
Small quantities of protactinium are used in advanced nuclear chemistry education and training programs for nuclear scientists and technicians. Its relatively long half-life (protactinium-231: about 32,570 years) makes it suitable for controlled laboratory experiments studying radioactive decay and nuclear chemistry principles.
Protactinium research contributes to understanding the stability of superheavy elements and predicting the properties of yet-undiscovered elements. Studies of protactinium's chemical and nuclear properties help validate theoretical models used in predicting the behavior of elements beyond the current periodic table.
The primary modern use of protactinium is in radiometric dating of marine sediments and geological samples. The Pa-231/U-235 dating method is particularly valuable for dating materials between 10,000 and 300,000 years old, filling a crucial gap in geochronological techniques.
Scientific institutions use protactinium for nuclear chemistry research, studying actinide behavior, and training nuclear scientists. Its properties make it an important reference element for understanding the entire actinide series and developing techniques for handling other radioactive materials.
Protactinium ratios in ocean sediments help reconstruct past climate conditions and ocean circulation patterns. Scientists use Pa-231/Th-230 ratios to study changes in Atlantic Ocean circulation and understand how climate has varied over geological timescales.
Researchers use protactinium isotopes to investigate nuclear decay mechanisms and test theoretical models of nuclear structure. These studies contribute to our fundamental understanding of atomic nuclei and radioactive decay processes.
Where it comes from
1.40e-6 mg/kg of Earth's crust · more abundant than 29% of elements
Protactinium occurs in nature in extremely small quantities within uranium ores, with concentrations of about 0.3 to 3 parts per million (average crustal rock holds only a few parts per trillion). It forms as an intermediate product in the uranium-235 decay chain, where uranium-235 decays through several steps to eventually produce protactinium-231.
Natural protactinium can be found wherever uranium deposits exist, including major uranium-producing regions in Canada, Australia, Kazakhstan, Niger, and the United States. However, the quantities are so minute that extraction from natural sources is impractical for any commercial purpose.
Marine sediments contain measurable traces of protactinium-231 deposited over geological time through natural uranium decay. These deposits, while extremely dilute, are sufficient for radiometric dating applications and paleoclimatology research. In sediment cores, the ratio of protactinium-231 to thorium-230 dates sediments up to about 175,000 years old and has helped reconstruct how North Atlantic waters moved as the last Ice Age glaciers melted.
Most protactinium-231 used for research has been extracted from spent nuclear fuel and uranium-processing waste; the UKAEA recovered about 127 grams from 60 tonnes of waste in 1961. Separated in a 12-stage process, that batch was for many years the world's only significant supply of protactinium, provided to laboratories for scientific studies.
Natural protactinium consists almost entirely of the Pa-231 isotope (half-life: about 32,570 years), with traces of shorter-lived isotopes from various decay chains. Spread through the crust at about 1.4 parts per trillion, natural protactinium adds up to at least hundreds of thousands of tonnes, far too dispersed to extract.
Handling
Protactinium is radioactive. It has no stable isotope — every nucleus decays. Handling requires appropriate shielding and licensing.
DANGER: Protactinium-231 is radioactive, about one twentieth as active per gram as radium-226, and is dangerous mainly if inhaled or swallowed. Protactinium-231, an alpha emitter with a half-life of about 32,570 years, is a long-term contamination hazard, while the short-lived protactinium-233 and protactinium-234 emit beta particles.
Protactinium exposure can cause acute radiation syndrome, increased cancer risk, and severe tissue damage. Alpha particles from protactinium decay are particularly dangerous if inhaled or ingested, as they can cause significant cellular and DNA damage in internal organs.
Protactinium must only be handled in specialized nuclear facilities with appropriate shielding, remote handling equipment, and extensive safety protocols. Personnel require advanced radiation safety training and continuous health monitoring due to the serious hazards involved.
Any suspected protactinium contamination requires immediate evacuation and specialized decontamination procedures. Emergency response must involve qualified radiation safety professionals and nuclear medicine specialists trained in handling severe radioactive contamination incidents.
Quick answers
Protactinium (symbol Pa) is element 91 on the periodic table, an actinide in period 7. One of the rarest and most expensive natural elements — extracting 125 g once cost half a million dollars. At room temperature it is a solid, and it is radioactive.
[Rn] 5f² 6d¹ 7s², giving 7 occupied shells with the distribution 2, 8, 18, 32, 20, 9, 2.Protactinium melts at 1845 K (1571.9 °C) and boils at 4300 K (4026.9 °C).
The standard atomic weight of protactinium is 231.036 u. That is a weighted average across its naturally occurring isotopes, which is why it is rarely a whole number.
The most common protactinium atom, Pa-231, has 140 neutrons: its mass number, 231, minus its atomic number, 91. It makes up 100% of natural protactinium. The classroom shortcut, rounded atomic mass (231.036) minus atomic number, gives the same 140.
Protactinium has a density of 15.37 g/cm³. Water is 1.0 g/cm³, so a block of protactinium is about 15.4× heavier.
Protactinium has a Pauling electronegativity of 1.5. The scale runs from 0.70 (francium, the least greedy for electrons) to 3.98 (fluorine, the most). Values in this middle band tend to form covalent rather than strongly ionic bonds.
Protactinium was discovered in 1913 by Fajans & Gohring. Origin of the name: Greek protos + actinium, "before actinium".
Protactinium makes up about 1.40e-6 mg/kg of the Earth's crust — genuinely rare, which is why it is expensive.
Yes. Protactinium has no stable isotope — every one of its nuclei decays. Trace amounts occur naturally as decay products of heavier elements.