91231.04PaProtactinium
Protactinium (Pa) — element 91actinide · period 7
Protactinium is a chemical element with the symbol Pa and atomic number 91. It is an actinide in period 7 of the periodic table (the f-block), with a standard atomic mass of 231.04 u. At room temperature (25 °C) protactinium is a solid.
💡 Known for: One of the rarest and most expensive naturally occurring elements, used mainly in research.
# Protactinium properties
| Symbol | Pa |
|---|---|
| Atomic number | 91 |
| Atomic mass | 231.04 u |
| Category | actinide |
| Group | — (f-block, no group number) |
| Period | 7 |
| Block | f-block |
| Electron configuration | [Rn] 5f² 6d¹ 7s² |
| Full configuration | 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 5f² 6s² 6p⁶ 6d¹ 7s² |
| Electrons per shell | 2, 8, 18, 32, 20, 9, 2 |
| Protons / electrons | 91 / 91 |
| State at 25 °C | solid |
| Melting point | 1841 K (1567.8 °C, 2854.1 °F) |
| Boiling point | 4300 K (4026.8 °C, 7280.3 °F) |
| Density | 15.37 g/cm³ |
| Electronegativity (Pauling) | 1.5 |
| First ionization energy | 5.89 eV |
| Atomic radius | 180 pm |
| Discovered | 1913 |
# Electron configuration of protactinium
The electron configuration of protactinium is [Rn] 5f² 6d¹ 7s², written out in full as 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 5f² 6s² 6p⁶ 6d¹ 7s². Its 91 electrons fill 7 shells as 2, 8, 18, 32, 20, 9, 2, so the outermost shell holds 2 electrons.
# Related elements
ThThoriumprevious elementUUraniumnext elementAcActiniumactinideNpNeptuniumactinidePuPlutoniumactinideAmAmericiumactinideCmCuriumactinideBkBerkeliumactinideCfCaliforniumactinideEsEinsteiniumactinide
# Protactinium FAQ
What is the atomic number of protactinium?
The atomic number of protactinium is 91: every protactinium atom has 91 protons in its nucleus. Its symbol is Pa.
What is the atomic mass of protactinium?
The standard atomic mass of protactinium is 231.04 u.
What is the electron configuration of protactinium?
Protactinium's electron configuration is [Rn] 5f² 6d¹ 7s² (1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 5f² 6s² 6p⁶ 6d¹ 7s²), with 2, 8, 18, 32, 20, 9, 2 electrons per shell.
Is protactinium a metal?
Yes. Protactinium is an actinide, one of the metals.
What is the melting point of protactinium?
Protactinium melts at 1841 K (1567.8 °C, 2854.1 °F) and boils at 4300 K (4026.8 °C, 7280.3 °F).
What is protactinium used for / known for?
One of the rarest and most expensive naturally occurring elements, used mainly in research.