Protactinium
Atomic Data
| Atomic Number | 91 |
| Symbol | Pa |
| Atomic Weight | 231.04 u |
| Density (STP) | 15.37 g/cm³ |
| Melting Point | 1567.85 °C (1841 K) |
| Boiling Point | 4026.85 °C (4300 K) |
| Electronegativity | 1.5 (Pauling) |
| Electron Config. | 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 5f2 6s2 6p6 6d1 7s2 |
| Oxidation States | +4, +5 |
| Phase at STP | Solid |
| Category | Actinoid |
| Period / Group | 7 / None |
| CAS Number | 7440-13-3 |
Electron Configuration
[Rn] 5f2 6d1 7s2
| Shell | n | Subshell | Electrons | Cumulative |
|---|---|---|---|---|
| K | 1 | 1s | 2 | 2 |
| L | 2 | 2s | 2 | 4 |
| L | 2 | 2p | 6 | 10 |
| M | 3 | 3s | 2 | 12 |
| M | 3 | 3p | 6 | 18 |
| M | 3 | 3d | 10 | 28 |
| N | 4 | 4s | 2 | 30 |
| N | 4 | 4p | 6 | 36 |
| N | 4 | 4d | 10 | 46 |
| N | 4 | 4f | 14 | 60 |
| O | 5 | 5s | 2 | 62 |
| O | 5 | 5p | 6 | 68 |
| O | 5 | 5d | 10 | 78 |
| O | 5 | 5f | 2 | 80 |
| P | 6 | 6s | 2 | 82 |
| P | 6 | 6p | 6 | 88 |
| P | 6 | 6d | 1 | 89 |
| Q | 7 | 7s | 2 | 91 |
| Total | 91 | 91 | ||
Isotopes of Protactinium
Protactinium is monoisotopic: ²³¹Pa is its only naturally occurring stable isotope, accounting for 100% of all natural Protactinium.
| Isotope | Symbol | Protons | Neutrons | Abundance | Stability |
|---|---|---|---|---|---|
| Protactinium-231 | ²³¹Pa | 91 | 140 | trace | Stable |
Abundance & Occurrence
Protactinium is present in Earth's crust at approximately trace amounts by mass and at approximately trace amounts by mass throughout the universe.
Earth's Crust (ppm by mass)
Universe (ppm by mass)
Discovery & History
Read more about the discovery of the periodic table of elements →
Safety & Handling
- Radioactivity: Protactinium-231 (t½ = 32,760 years, alpha emitter) is highly radiotoxic; it concentrates in the kidneys and skeleton if absorbed, and its long half-life means contamination is persistent.
- Extreme rarity and handling context: Only about 125 g of Pa-231 exists in the world (at UKAEA); macroscopic protactinium handling occurs only at a handful of facilities globally, all operating under strict nuclear regulatory authority oversight.
- Chemical toxicity: Protactinium compounds are chemically toxic in addition to radiological hazards; the limited data available suggests kidney and liver as primary target organs for chemical toxicity.
- Containment: All Pa work requires shielded glove boxes, full contamination monitoring, and dosimetry; waste disposal requires specialist nuclear waste management.
Protactinium in the Real World
Real-World Uses
- Ocean sediment and coral dating: The uranium-protactinium (²³⁸U/²³¹Pa) dating system is used to determine the ages of deep-sea sediment cores and coral reefs from approximately 10,000 to 150,000 years ago, providing a record of past ocean circulation and sea level changes.
- Nuclear reactor physics research: Protactinium-231 is an intermediate in the thorium fuel cycle and is studied in reactor physics calculations; its neutron capture properties must be accounted for in the design of thorium-based reactors.
- Scientific study of actinide chemistry: Protactinium's complex oxidation state chemistry (Pa³⁺, Pa⁴⁺, Pa⁵⁺) and its position bridging light and heavy actinides make it a subject of fundamental study in actinide solution chemistry and coordination chemistry.
Downloadable Resources
Free periodic table reference sheets for classrooms, study sessions, and laboratory use.
Frequently Asked Questions
What is protactinium used for?
Protactinium has no significant practical uses. Protactinium-231 is used in scientific research for geochronology and oceanography: the ratio of Pa-231 to thorium-230 in marine sediments can be used to reconstruct past ocean circulation patterns. Due to its rarity, toxicity, and radioactivity, protactinium remains primarily a subject of scientific study rather than a commercially useful element.
How rare is protactinium?
Protactinium is one of the rarest naturally occurring elements, present in uranium ores at concentrations of about 1 part per million relative to uranium. The total amount in Earth's crust is estimated at about 3 tonnes globally. Obtaining even a few grams required the processing of 60 tonnes of uranium ore residue, as was done by the UK Atomic Energy Authority in the 1960s to obtain 125 grams of Pa-231 for research.
How was protactinium discovered?
Protactinium was partially discovered in 1913 by Kasimir Fajans and Otto Gohring, who identified the short-lived isotope Pa-234m in uranium decay chains and named it 'brevium'. The longer-lived and more important Pa-231 was discovered in 1917–1918 independently by Otto Hahn and Lise Meitner in Germany, and by Frederick Soddy and John Cranston in the UK. The name protactinium ('before actinium') reflects that it decays to actinium-227.
Why is protactinium important in oceanography?
Protactinium-231 and thorium-230 are both produced at known rates by radioactive decay of uranium dissolved in seawater, but they are removed from the water column by settling particles at different rates. By measuring the Pa-231/Th-230 ratio in ancient ocean sediments, oceanographers can reconstruct past changes in ocean circulation: particularly the strength of the Atlantic Meridional Overturning Circulation (AMOC), which affects global climate. This proxy is one of the main tools for understanding past ice age climates.