Protactinium (Pa)
Protactinium (symbol Pa) is the chemical element with atomic number 91: an actinide, solid at 0 °C, radioactive, with an atomic mass of 231.04 u. Discovered in 1913 (Kasimir Fajans and Oswald Göhring).
Protactinium is an extremely rare radioactive metal found in uranium ores. Its name means “before actinium” because it decays into actinium. Its most stable isotope, protactinium-231, has a half-life of about 33,000 years. It has no industrial use, but geologists rely on it to date ocean sediments.
Key facts
- Atomic number
- 91
- Symbol
- Pa
- French name
- Protactinium
- Atomic mass
- 231.04 u
- Molar mass
- 231.04 g/mol
- Family
- Actinides
- Position
- period 7, f-block
- State at 0 °C
- Solid
- Melting point
- 1,567.9 °C
- Boiling point
- 4,026.9 °C
- Density
- 15.37 g/cm³
- Electronegativity (Pauling)
- 1.5
- First ionization energy
- 568 kJ/mol
- Atomic radius
- 180 pm
- Electron configuration
- [Rn] 5f² 6d¹ 7s²
- Abundance in Earth's crust
- 0.000001 ppm (84th most abundant element)
- Radioactive
- yes
- Discovery
- 1913, Kasimir Fajans and Oswald Göhring
Origin of the name
From the Greek prōtos, first: “the one before actinium.”
What does protactinium look like?
A shiny, silvery-gray metal that keeps its luster for a while in air. It is dense (about 15 g/cm³) and becomes a superconductor at very low temperatures.
Where is protactinium found?
It occurs in uranium ores at a few tenths of a gram per tonne of uranium. It is born from the decay of uranium-235.
Natural sources
- Traces in uraninite (pitchblende)
How is protactinium produced?
It is not made industrially. The largest batch ever obtained, about 125 grams, was extracted in the 1960s in the United Kingdom from some sixty tonnes of uranium processing waste, through a process of about a dozen steps.
What is protactinium used for?
- Basic research
- Dating marine sediments (protactinium-231 / thorium-230 ratio)
- Studying past ocean circulation
In everyday life
- None
Role in living things
No biological role. If ingested, it would settle mainly in the bones.
Hazards of protactinium
Very toxic and highly radioactive, an alpha emitter. It calls for precautions comparable to those for plutonium.
Cosmic origin
- None of it survives from the formation of the Solar System: it is constantly being created by the radioactive decay of uranium and thorium.
History of protactinium
In 1913, Kasimir Fajans and Oswald Göhring identified a very short-lived isotope they named “brevium” (from the Latin brevis, brief). In 1917–1918, Otto Hahn and Lise Meitner in Berlin, along with Frederick Soddy and John Cranston in Scotland, discovered the far more stable isotope 231. The name protoactinium, shortened to protactinium in 1949, won out.
Did you know?
- The British 125 g stockpile long supplied most of the world's research on this element.
- The 1913 “brevium” lived up to its name: that isotope, protactinium-234m, has a half-life of just over a minute.
- Mendeleev had saved an empty slot for it, below tantalum: he called it “eka-tantalum.”
- It was one of the first discoveries of Lise Meitner, who would later explain nuclear fission.
Isotopes of protactinium
No stable isotopes; protactinium-231 exists in traces in uranium ores. It is one of the rarest and most expensive natural elements.
- protactinium-231: 100% of natural protactinium, radioactive, half-life 32,800 years. Used to date ocean sediments, by comparing it with thorium-230.
- protactinium-233: radioactive, half-life 27 days. The intermediate step between thorium-232 and uranium-233 in thorium reactors.
- protactinium-234: radioactive, half-life 6.7 h
- protactinium-234m: radioactive, half-life 1.16 min. The element's first identified isotope, in 1913, hence its old name “brevium” (brief).
Related elements
Same family (actinides): Actinium (Ac), Thorium (Th), Uranium (U), Neptunium (Np), Plutonium (Pu), Americium (Am), Curium (Cm), Berkelium (Bk), Californium (Cf), Einsteinium (Es), Fermium (Fm), Mendelevium (Md), Nobelium (No), Lawrencium (Lr).