Protactinium is a chemical element with symbol Pa and atomic number 91. It is a dense, silvery-gray metal which readily reacts with oxygen, water vapor and inorganic acids. It forms various chemical compounds where protactinium is usually present in the oxidation state +5, but can also assume +4 and even +2 or +3 states.
Three reasons Protactinium stands out
1
Radioactive metal, used in nuclear reactors and some research applications.
2
Its name literally means "before actinium," as it decays radioactively to produce actinium.
3
It is found only in tiny trace amounts as a decay product inside uranium ores.
A rare pause in uranium’s decay
Protactinium sits between uranium and actinium in a decay chain, which is exactly what its name announces. It is scarce, radioactive, and difficult to handle, so it has few uses outside research. Its complex electron behavior gives chemists a challenging bridge between lighter and heavier actinides.
Recognizing Protactinium in Elementle
If Protactinium is the mystery element, guesses below atomic number 91 point up and guesses above it point down. Its symbol is Pa: a green letter matches the same position in that symbol, while a yellow letter belongs in another position.
The family clue turns green for actinide guesses. Compare Thorium (Th), number 90 and Uranium (U), number 92: they share the family clue, but their atomic numbers and symbols help you tell them apart.
Values are reference data, not a specification for every temperature, pressure, or allotrope. Unknown properties are left unavailable. Superheavy-element predictions remain uncertain.
The introduction and original editorial story are adapted from Wikipedia contributors via Periodic-Table-JSON. Element descriptions and their adaptations are available under CC BY-SA 3.0. Numerical attributes use the PubChem periodic-table dataset, retrieved September 22, 2026.