Element 91 · Actinide
Protactinium (Pa) — Element 91
Proto-actinium
Protactinium (Pa) is an actinide in Period 7 (f-block) of the periodic table with atomic number 91, standard atomic weight 231.04 u, and ground-state electron configuration [Rn] 5f² 6d¹ 7s². At standard temperature and pressure it is classified as a solid with 5 valence electrons and common oxidation states +5, +4.
Research only
What are the atomic properties and physical constants of Protactinium?
Reference constants compiled from IUPAC standard atomic weights, the NIST Atomic Spectra Database, and the CRC Handbook.
- Atomic number (Z)
- 91
- Standard atomic mass
- 231.04 u
- Chemical family
- Actinide
- Ground-state configuration
- [Rn] 5f² 6d¹ 7s²
- Electrons per shell
- 2, 8, 18, 32, 20, 9, 2
- Valence electrons
- 5
- Common ion charges / oxidation states
- +5, +4
- Phase at STP
- Solid
- Pauling electronegativity
- 1.50 Pauling
- First ionization energy
- 568 kJ/mol
- Atomic radius
- —
- Melting point
- 1,568 °C
- Boiling point
- 4,027 °C
- Discovery year
- 1913
- Table coordinate
- Period 7, f-block
- Radioactivity
- Radioactive (no stable isotopes)
How are electrons arranged across Protactinium’s shells?
A neutral Protactinium atom contains 91 electrons distributed across 7 occupied principal energy shells ([Rn] 5f² 6d¹ 7s²). The table below compares Protactinium’s shell occupancy against each shell’s theoretical 2n² capacity.
| Principal Shell (n) | Spectroscopic Letter | Electrons in Pa | Max Shell Capacity (2n²) |
|---|---|---|---|
| n = 1 | K | 2 e⁻ | 2 e⁻ |
| n = 2 | L | 8 e⁻ | 8 e⁻ |
| n = 3 | M | 18 e⁻ | 18 e⁻ |
| n = 4 | N | 32 e⁻ | 32 e⁻ |
| n = 5 | O | 20 e⁻ | 50 e⁻ |
| n = 6 | P | 9 e⁻ | 72 e⁻ |
| n = 7 | Q | 2 e⁻ | 98 e⁻ |
Frequently Asked Questions about Protactinium (Pa)
What is the atomic mass and atomic number of Protactinium (Pa)?
Protactinium (Pa) has atomic number Z = 91 (91 protons in its nucleus) and an IUPAC standard atomic weight of 231.04 u. It sits in Period 7 in the f-block and is classified as an actinide.
What is the ground-state electron configuration of Protactinium?
The neutral ground-state electron configuration of Protactinium (Z = 91) is [Rn] 5f² 6d¹ 7s². Across its 7 occupied principal shells (n = 1 to n = 7), the electrons per shell are 2, 8, 18, 32, 20, 9, 2, with 5 valence electrons participating in bonding.
What are the common ion charges and electronegativity of Protactinium?
Protactinium commonly forms +5, +4 oxidation states or ion charges in compounds. Its Pauling electronegativity is 1.50 and its first ionization energy is 568 kJ/mol.
When was Protactinium discovered and what is it used for?
Protactinium was discovered in 1913 (Proto-actinium). Research only
How does Protactinium compare to neighboring elements?
Other actinide elements: