Protactinium
Protactinium is a chemical element with symbol Pa and atomic number 91. It is a dense, silvery-gray metal which readily reacts with oxygen, water vapor and inorganic acids. It forms various chemical compounds where protactinium is usually present in the oxidation state +5, but can also assume +4 and even +2 or +3 states.
Identity
- Atomic mass
- 231.036 u
- Category
- Actinides
- Group
- 3
- Period
- 7
- Block
- f
- Oxidation States
- +4, +5
- Phase (25 °C)
- Solid
- Appearance
- bright, silvery metallic luster
Electronic Structure
- Electron configuration
- 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 5f2 6d1
- Short configuration
- [Rn] 5f2 6d1 7s2
- Electrons per shell
- 2, 8, 18, 32, 20, 9, 2
- Electronegativity (Pauling)
- 1.50
- Electron affinity
- 53.0 kJ/mol
- 1st ionization energy
- 568 kJ/mol
Physical properties
- Melting point
- 1567.8 °C (1841 K)
- Boiling point
- 4026.8 °C (4300 K)
- Density
- 15.37 g/cm³
- Molar heat capacity
- —
Chemical behavior of protactinium
Protactinium has five outer electrons ([Rn] 5f² 6d¹ 7s²) and acts as +5, with a stable +4 in the dioxide and halides. Its electronegativity is low (1.50) and it is dense, shiny, and extremely rare. All of its chemistry is studied in shielded micrograms: it is so radioactive that it heats and decomposes its own compounds.
Reactivity
4 Pa + 5 O₂ → 2 Pa₂O₅
In air it coats itself with the very stable white pentoxide. When heated it burns to the +5 oxide, which resists concentrated acids.
Pa + 4 HF → PaF₄ + 2 H₂
With hydrofluoric acid it gives an insoluble tetrafluoride, useful for separating it from uranium and thorium in reprocessing.
Pa₂O₅ + 6 HCl → 2 PaOCl₃ + 3 H₂O
With hydrochloric acid it forms complex oxychlorides, the basis of its separation by solvent extraction in hot cells.
Pa + 5 F₂ → PaF₅
With excess fluorine it forms the volatile pentafluoride, which sublimes and allows purification by fluoride distillation.
Notable compounds
| Compound | Formula | Use or importance |
|---|---|---|
| Protactinium pentoxide | Pa₂O₅ | Stable form and Pa-Th dating |
| Protactinium tetrafluoride | PaF₄ | Uranium and thorium separation |
| Protactinium pentafluoride | PaF₅ | Purification by volatility |
| Protactinium dioxide | PaO₂ | Experimental fuels |
| Protactinium oxychloride | PaOCl₃ | Radiochemical intermediate |
Electron shell distribution
View in the atom builderIsotopes
Protactinium-231 (²³¹Pa, 32,760 years) dominates the uranium-235 chain and is the reference isotope for marine dating. Protactinium-233 (²³³Pa, 27 days) is the mandatory bridge between thorium-233 and uranium-233 in thorium cycles. Protactinium-234m (234m, 1.17 minutes) flags uranium leaks in safeguards.
Occurrence and production
It occurs in trace amounts in pitchblende, on the order of 0.3 ppm of the uranium content: about 100 tonnes in all accessible crust. All of it comes from the decay of uranium-235. It is extracted from uranium residues or spent fuel by solvent extraction in licensed facilities.
History and origin of the name
Kasimir Fajans and Oswald Göhring glimpsed it in 1913 as brevium, for its short life; Hahn and Meitner isolated Pa-231 in 1917 and named it protactinium, because it engenders actinium when it decays. The priority dispute lasted for years. Its rarity kept it in display cases until oceanography turned it into a climate clock.
Applications and relevance of protactinium
Uses of the element
As a metal it is barely used because of its scarcity and radiation; its value is scientific: Pa-231 targets calibrate alpha spectrometers and support superheavy actinide studies.
Uses of its compounds
The 231Pa/230Th pair dates sediments and reconstructs Atlantic circulation over the past 175,000 years. Intermediate Pa-233 monitors U-233 breeding in thorium reactors.
Health, safety and environment
Biological role
It has no biological function and is highly radiotoxic: it accumulates in bone, kidney, and liver with a long biological half-life. Any intake requires medical monitoring and experimental chelation.
Safety
It is handled in glove boxes with shielding, ventilation, and alpha-beta-gamma monitors. Work is done in micrograms; it does not exist in consumer products or household detectors.
Environment
There are no routine spills: all inventory is traceable and waste is vitrified with spent fuel. Its measurement in sediments is a climate tool, not pollution.
Frequently asked questions about Protactinium
What is protactinium used for?
Protactinium has very few applications outside scientific research. Its isotope protactinium-231 is used in geochronology and paleoceanography to date marine sediments and study changes in ocean circulation over the past several thousand years. It is also used in nuclear research and in decay-chain studies. Its scarcity, radiotoxicity, and cost rule out broad commercial uses.
Is protactinium dangerous?
Yes. Protactinium is highly radioactive and toxic, and it has no known biological function. Its main risk comes from radiation and from contamination through inhalation, ingestion, or contact with dust. For this reason it is handled in glove boxes, with shielding, controlled ventilation, and strict radiation protection protocols. It is not present in consumer products or household applications.
What does it mean that protactinium is an actinide?
Protactinium belongs to the actinides, the series of radioactive metallic elements that runs from actinium to lawrencium. It shares with them the tendency to form compounds with variable oxidation states and to appear in the decay chains of uranium and thorium. Its name comes from the Greek protos, "first", because protactinium-231 decays into actinium-227.
Where does protactinium for research come from?
Protactinium can be extracted from uranium minerals, though in very small quantities, or recovered from spent nuclear fuel rods. In both cases, radiochemical separation and purification processes in licensed facilities are required. Because of its radiotoxicity and handling difficulties, it is produced and used in tiny amounts, mainly for geochronology, nuclear research, and environmental tracer studies.
Why is protactinium so scarce and expensive?
Protactinium occurs in tiny amounts in uranium minerals and forms as part of natural radioactive chains. Although protactinium-231 has a long half-life, its concentration in Earth's crust is very low and its separation is difficult. In addition, it must be handled as radioactive material in specialized facilities. This combination of scarcity, radiotoxicity, and complexity of production explains its high price.
How does protactinium help date marine sediments?
The ratio between the isotopes protactinium-231 and thorium-230 in sediments allows scientists to estimate the age of geological layers and reconstruct changes in ocean circulation. Both isotopes form from the decay of uranium in seawater and settle at different rates. By comparing their proportions, scientists can date sediments up to about 175,000 years old and study past climate variations.
Is protactinium found in smoke detectors?
Not as an intentional component. Household smoke detectors use americium-241, not protactinium. However, americium decays over time and can produce trace amounts of protactinium as a daughter product. The amount is insignificant and stays contained within the detector's sealed source. Therefore a smoke detector should not be considered to contain usable protactinium or to pose any relevant exposure.
Main sources consulted: PubChem, Royal Society of Chemistry, NIST, IUPAC. Last revision: November 2026.