Protactinium (Pa): Electron Configuration, Properties and Uses
Protactinium (Pa): Electron Configuration, Properties and Uses
Electron configuration [Rn] 7s2 5f2 6d1 · f-block, actinide series · period 7 · Ar 231.04
Protactinium data
- Atomic number
- 91
- Relative atomic mass
- 231.04
- Group
- – (actinide series)
- Period
- 7
- Block
- f
- Category
- Actinide
- Electron configuration
- [Rn] 7s2 5f2 6d1
- Full configuration
- 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 5f2 6d1
- Electrons per shell
- 2, 8, 18, 32, 20, 9, 2 (K L M N O P Q)
- Valence electrons
- 2 (in the outer shell)
- Electronegativity (Pauling)
- 1.5
- First ionisation energy
- 5.89 eV
- Atomic radius (van der Waals)
- 243 pm
- State at room temperature
- Solid
- Melting point
- 1572 °C
- Boiling point
- no data
- Density
- 15.37 g/cm³
- Common oxidation states
- +5, +4
- Discovered
- 1913
Source: PubChem (NIH, accessed 13 Nov 2026); atomic mass: IUPAC standard atomic weight.
Electron shell diagram
K 2 · L 8 · M 18 · N 32 · O 20 · P 9 · Q 2
Orbital diagram (outer electrons)
About protactinium
With atomic number 91, protactinium (Pa) has 91 protons in its nucleus and 91 electrons as a neutral atom. It belongs to the actinide series in period 7 (f-block), usually shown below the main table. The element was discovered in 1913. Protactinium is a solid at room temperature, melting at 1572 °C.
Protactinium electron configuration
The electron configuration of protactinium is 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 5f2 6d1, or in short [Rn] 7s2 5f2 6d1. Its electrons occupy 7 shells: K = 2, L = 8, M = 18, N = 32, O = 20, P = 9, Q = 2. As an actinide, protactinium fills the 5f subshell. Its outer 7s subshell holds 2 electrons; valence electrons for this series are not assigned by the usual group rule. This is an exception to the Aufbau rule: the usual filling order predicts [Rn] 7s2 5f3, but the 5f and 6d subshells are so close in energy that electrons occupy 6d. When it forms cations, electrons leave the outermost subshell first: Pa4+ = [Rn] 5f1.
Filling order: 1s2(2) → 2s2(4) → 2p6(10) → 3s2(12) → 3p6(18) → 4s2(20) → 3d10(30) → 4p6(36) → 5s2(38) → 4d10(48) → 5p6(54) → 6s2(56) → 4f14(70) → 5d10(80) → 6p6(86) → 7s2(88) → 5f2(90) → 6d1(91)
Uses of protactinium
In industry and the laboratory, protactinium turns up in many places, for example research, including dating ocean sediments with the Pa-231 and Th-230 pair.
Safety
Protactinium is radioactive. Using, storing or transporting radioactive material requires a licence from the national regulator (BAPETEN in Indonesia). Always read the safety data sheet (SDS) before working with a chemical.
Other actinides
Frequently asked questions
What is the electron configuration of protactinium?
The electron configuration of protactinium (Pa, atomic number 91) is 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 5f2 6d1, or [Rn] 7s2 5f2 6d1 in short. By shell: 2, 8, 18, 32, 20, 9, 2.
How many valence electrons does protactinium have?
Protactinium has 2 electrons in its outer shell. For lanthanides and actinides, valence electrons are not assigned by the usual group rule.
Which group and period is protactinium in?
Protactinium is in the actinide series, period 7, f-block.
What is the atomic mass of protactinium?
The relative atomic mass of protactinium is 231.04. Use it to calculate molar mass with the molar mass calculator.
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