Protactinium (Pa)
Actinide · Radioactive
Protactinium is one of the rarest and most expensive natural elements — a radioactive metal sandwiched between thorium and uranium. Oceanographers use its slow decay to date deep-sea sediments.
Position on the periodic table
Atomic properties
| Atomic number | 91 |
|---|---|
| Atomic mass | 231.036 amu |
| Electron configuration | 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 5f2 6d1 |
| Noble gas shorthand | [Rn] 5f2 6d1 7s2 |
| Electrons per shell | 2, 8, 18, 32, 20, 9, 2 |
| Valence electrons (outer shell) | 2 |
| Common oxidation states | +5, +4 |
| Electronegativity (Pauling) | 1.5 |
| Covalent radius (approx.) | 200 pm |
| First ionization energy | 568 kJ/mol |
| Electron affinity | 53 kJ/mol |
Physical properties
| State at 25 °C | Solid |
|---|---|
| Density | 15.37 g/cm³ |
| Melting point | 1567.8 °C (1841 K) |
| Boiling point | 4026.8 °C (4300 K) |
| Appearance | bright, silvery metallic luster |
| Radioactive | Yes |
| Origin | Occurs naturally |
Electron configuration of protactinium
Protactinium's ground-state electron configuration is 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 5f2 6d1, usually shortened to [Rn] 5f2 6d1 7s2. Its electrons occupy 7 shells (2, 8, 18, 32, 20, 9, 2), placing it in period 7 of the f-block. Explore it interactively in the electron configuration calculator.
Uses of protactinium
- Scientific research and ocean-sediment dating only
Biological role: None.
Occurrence: Trace amounts in uranium ores.
Common compounds of protactinium
Pa2O5
Protactinium(V) oxide
History and discovery
Discovered: 1913 — William Crookes. Name origin: From “proto-actinium” — it decays into actinium.
Identified in 1913 by Fajans and Göhring (as “brevium”); Lise Meitner and Otto Hahn found the long-lived isotope in 1917.
Safety notes
Highly radiotoxic; research facilities only.
Educational context only — always follow your school's laboratory rules and never handle chemicals without proper supervision. See our disclaimer.
Practice questions
Quick practice: Protactinium
1. What is the chemical symbol of Protactinium?
2. What is the atomic number of Protactinium?
3. Which category does Protactinium belong to?
4. What is the state of Protactinium at room temperature?
5. Which period of the periodic table is Protactinium in?
Protactinium FAQs
What is the atomic number of protactinium?
Protactinium's atomic number is 91 — every protactinium atom has 91 protons in its nucleus.
What is the symbol for protactinium?
Pa. From “proto-actinium” — it decays into actinium.
Is protactinium a metal, nonmetal, or metalloid?
Protactinium is classified as a actinide.
What state is protactinium at room temperature?
At about 25 °C, protactinium is a solid. It melts at 1567.8 °C.
What is protactinium used for?
Key uses include: scientific research and ocean-sediment dating only.
Related elements
Keep working with this element
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