Protactinium Pa

Discovery

When Dmitri Mendeleev arranged his periodic table in 1871, he left a gap between thorium and uranium for an element nobody had yet seen. The first occupant was found in 1913 by Kasimir Fajans and Oswald Göhring in Karlsruhe. Studying the decay chain of uranium, they identified a fleeting isotope that vanished within about a minute (now known as protactinium-234m) and called it 'brevium', from the Latin for short.

In 1917–1918 Otto Hahn and Lise Meitner in Berlin found a far longer-lived isotope, protactinium-231. Frederick Soddy and John Cranston in Britain reached the same result independently at about the same time.

Preparing a pure sample took decades. Aristid von Grosse isolated the oxide in 1927 and the metal in 1934. In the early 1960s the United Kingdom Atomic Energy Authority worked through roughly 60 tonnes of uranium-processing residue to recover a little over 100 grams.

Origin of the name

Hahn and Meitner combined the Greek protos (first, before) with actinium to make 'protoactinium', because protactinium-231 decays by alpha emission into actinium-227; it is, in effect, actinium's parent. In 1949 IUPAC dropped one letter to make the name easier to say, giving 'protactinium'. The symbol Pa is formed from the first two letters.

Main uses

• Fundamental research: Nearly all purified protactinium goes to laboratories. Sitting where the 5f electron shell begins to fill, it is a key test case for understanding how actinide chemistry changes across the series.
• Oceanography and climate science: Uranium dissolved in seawater produces protactinium-231 and thorium-230 at a steady rate, but the two stick to sinking particles differently. Measuring their ratio in sediment cores lets researchers reconstruct how vigorously the Atlantic's deep circulation flowed in past climates.
• Thorium reactor studies: In the thorium fuel cycle, thorium-232 captures a neutron and passes through protactinium-233 before becoming fissile uranium-233. Its behaviour matters for thorium reactor design.

Isotopes

Protactinium has no stable isotopes. The longest-lived, protactinium-231, has a half-life of about 32,800 years and occurs naturally as part of the uranium-235 decay series. Paired with thorium-230, it can date marine sediments and corals up to a few hundred thousand years old. Protactinium-233, with a half-life of about 27 days, is the crucial stepping stone in the thorium fuel cycle. The isotope Fajans first spotted, protactinium-234m, lasts only about 1.2 minutes on average before decaying.

In everyday life

You are extremely unlikely ever to encounter protactinium. A tonne of uranium ore holds well under a gram of it, although trace amounts are present in every uranium mineral and in the soil around such deposits. It is a strong alpha emitter and chemically toxic, and once inside the body it can lodge in bone, so laboratories handle it only in sealed glove boxes.