ElementsWhat is protactinium?
Protactinium is a rare, radioactive, silvery metal found in tiny amounts in uranium ores.
What is protactinium used for?
No uses outside research, such as dating ocean sediments.
Where the name comes from
From Greek words meaning 'before actinium', because it decays into actinium.
Key facts
- Symbol
- Pa
- Atomic number
- 91
- Atomic mass
- 231.03588 u
- Family
- Actinide
- Period
- 7
- Group
- none (f-block)
- Block
- f
- State at room temperature
- Solid
- Melting point
- 1,572 °C (1,845 K, 2,861 °F)
- Boiling point
- not measured
- Density
- 15.37 g/cm³
- Electrons per shell
- 2, 8, 18, 32, 20, 9, 2
- Electron configuration
- [Rn]7s2 5f2 6d1
- Common oxidation states
- +5, +4
- Electronegativity (Pauling)
- 1.5
- First ionisation energy
- 5.89 eV
- Electron affinity
- no value
- Atomic radius (van der Waals)
- 243 pm
- Discovered
- 1913
Place in the periodic table
Protactinium is element number 91. It sits in period 7 (row 7) of the periodic table, in the separate row of actinides drawn below the main table, and belongs to the f-block. Just before it comes thorium (90), and just after it comes uranium (92).
Family: Actinides.
Physical properties
At room temperature protactinium is a solid. Its melting point is about 1,572 °C. Its boiling point has not been measured reliably. Its density is 15.37 grams per cubic centimetre, so a one-litre block would weigh about 15.37 kilograms. By density it ranks 11th among the 96 elements with a measured value.
Inside the atom
An atom of protactinium has 91 protons in its nucleus and, when neutral, 91 electrons around it. The electrons are arranged in 7 shells: 2, 8, 18, 32, 20, 9, 2. The outermost shell holds 2 electrons. Written in full, the electron configuration is [Rn]7s2 5f2 6d1.
Its electronegativity on the Pauling scale is 1.5. This number shows how strongly an atom pulls shared electrons towards itself: fluorine, the strongest, is 3.98. Removing one electron from an atom takes 5.89 electronvolts (about 568 kilojoules per mole). One mole of protactinium weighs 231.03588 grams, so a single gram contains about 2.61 × 1021 atoms.
The ring picture is a simple model that only counts the electrons in each shell. Real atoms do not look like this.
Next steps
- Work out the weight of a compound that contains protactinium with the molar mass calculator.
- Compare it with every other element in the sortable list.
- Test yourself with the symbol quiz.
Figures on this page are from PubChem (U.S. National Library of Medicine), as downloaded on 11 November 2026. See About and data sources.