Protactinium (Pa)
“So scarce that the world’s entire purified stock — 125 grams — sits in one British laboratory: the element Mendeleev’s table foretold and two rival teams finally caught in 1913 and 1918.”
How its electrons are arranged
Computed from the aufbau principle, with known real-world exceptions applied automatically.
The notes
Characteristics
- Electron configuration [Rn] 5f² 6d¹ 7s² — period 7, f-block; among oxidation states from +3 to +5 it is +5 that rules, and Pa(V) stubbornly hydrolyses in water like an old transition metal rather than a lanthanide.
- Pa-231, half-life 32,760 years, is a fierce alpha emitter — each gram crackles through nearly 1,750 million decays every second — and specks of it rank among the most radiotoxic substances known.
- One of the rarest natural elements: pitchblende carries about one part per million, so the tonnes of ore the Curies processed hid only milligrams; no rich protactinium ore exists anywhere on Earth.
- With a density of 15.4 g/cm³ it outweighs lead, and its chemistry sits beside tantalum’s — exactly the slot Mendeleev had labelled “eka-tantalum” in his table forty years before anyone held a sample.
A dense, lustrous, silvery metal that slowly tarnishes in air — among the heaviest and rarest elements found in nature. It is malleable, attacked by strong acids, and below 1.4 K even turns superconducting, an astonishing trick for so radioactive an atom.
Where you meet it
✦ Uses
- Protactinium–thorium dating: the Pa-231 : Th-230 ratio sealed in deep-sea sediments works as a geological clock for the past 175,000 years or so, tracking ancient ocean circulation for palaeoclimate science.
- Oceanographic tracer: measuring dissolved protactinium against thorium in seawater lets scientists reconstruct how the Atlantic’s great overturning circulation behaved through the ice ages.
- Fundamental actinide research: its awkward, hydrolysis-happy chemistry helps chemists map the 5f series, and the UK’s 125-gram stock is lent out in milligram crumbs for exactly such studies.
- A hidden step in the thorium fuel cycle: 27-day Pa-233 is the intermediate that must be patiently held while it ripens into fissile U-233 — reactor designers must budget for its appetite for stray neutrons.
◍ Everyday examples
- Practically nowhere in daily life — and that is the point: every purified gram of protactinium on Earth lives inside one guarded British facility, making it vastly harder to obtain than gold.
- Documentaries about ice-age oceans lean on protactinium–thorium ratios hidden in mud cores hauled from the Atlantic seabed.
- Mendeleev’s 1871 “eka-tantalum” gap — retold in every periodic-law chapter, including NCTB chemistry — was this very element waiting 47 years to be caught.
History & name
In 1913 Kasimir Fajans and Oswald Göhring, working at Karlsruhe, caught a furious beta emitter living just 1.17 minutes in the uranium-238 decay chain and christened it “brevium” — too short-lived to be the real prize. That prize arrived in 1918, when two teams — Otto Hahn and Lise Meitner in Berlin, and Frederick Soddy with John Cranston in Glasgow — independently isolated the long-lived isotope protactinium-231, the missing parent of actinium. The name literally means “parent of actinium” (Greek protos + actinium), because Pa-231 alpha-decays into actinium-227. Not until 1961 did the UK Atomic Energy Authority tame chemistry vicious enough to wrest 125 grams of pure protactinium from 60 tonnes of spent reactor fuel — still the only sizeable stock of purified protactinium on Earth.