One gram per ten tons of rock. Protactinium is one of the rarest naturally occurring elements on Earth. This heavy radioactive metal hides inside uranium ore, with atomic number 91 and a density of 15.37 g/cm³ — heavier than lead. The entire world supply is less than one kilogram. Its price reaches $280 per gram.
The name means "before actinium," and for good reason: Pa-231 decays into actinium-227. The element was discovered in 1913 in Germany, when Fajans and Göhring found the short-lived Pa-234 among uranium decay products. The more stable Pa-231 was isolated only in 1927.
Protactinium is a shiny silvery-white metal that quickly tarnishes in air. Its melting point is 1,572 C, making it refractory. Chemically, it behaves like niobium and tantalum, which makes extraction from ore extremely difficult.
Practical uses are almost nonexistent — the element is simply too rare and radioactive. But scientists rely on it for dating marine sediments and studying ancient ocean circulation. The Pa/Th ratio in seafloor deposits reveals how Earth's climate worked thousands of years ago.
Protactinium is extremely radioactive and toxic. Pa-231 emits alpha particles and gamma rays with a half-life of 32,760 years. Inhaling even tiny amounts of dust causes severe internal irradiation and can lead to cancer. Work with protactinium is restricted to specialized laboratories with the highest level of radiation shielding. It must be stored in sealed lead containers at all times.
Protactinium is so rare that the entire Earth's crust contains only 1 gram per 10 tons of rock. That is a million times less than gold.
The name means "before actinium." Pa-231 decays into actinium-227. The element is literally named after its own offspring.
Isolating 125 grams of pure protactinium took British scientists 5 years and 60 tons of uranium waste. That remains the largest sample ever produced.
The world's entire protactinium supply is under 1 kilogram. At $280 per gram, it costs 4 times more than gold.
Pa-231 has a half-life of 32,760 years. Half of its atoms will decay over a span ten times longer than all of recorded human history.
Geochemists measure Pa/Th ratios in marine sediments to reconstruct ocean currents that flowed 20,000 years ago during the last ice age.
| Isotope | Mass (u) | Abundance | Half-life | Decay |
|---|---|---|---|---|
231Pa☢ | 231.035884 | 100.00% | 32760 years | α |
233Pa☢ | 233.040247 | synthetic | 26.975 days | β− |
Uranium decay products