91PaProtactinium231.03588 u
Protactinium (Pa)
Protactinium is an actinide with atomic number 91, in period 7, in the f-block of the periodic table. Its atomic mass is 231.03588 u and its electron configuration is [Rn] 5f² 6d¹ 7s². Solid at 25 °C.
Key facts
| Atomic number | 91 |
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| Atomic mass | 231.03588 u |
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| Category | Actinide |
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| Period | 7 |
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| Group | f-block, no group number |
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| Block | f |
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| Electron configuration | [Rn] 5f² 6d¹ 7s² |
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| Electrons per shell | 2, 8, 18, 32, 20, 9, 2 |
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| Electronegativity (Pauling) | 1.5 |
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| Oxidation states | +3, +4, +5common in bold |
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| State at 25 °C | Solid |
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| Melting point | 1845 K1571.85 °C |
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| Boiling point | 4273 K3999.85 °C |
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| Density | 15.37 g/cm³ |
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Atomic properties
| First ionization energy | 5.89 eV |
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| Electron affinity | --- |
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| Atomic radius | 161 pm |
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| Covalent radius | 200 pm |
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| Van der Waals radius | 243 pm |
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| Common ions | Pa⁵⁺, Pa⁴⁺ |
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| Protons, neutrons, electrons | 91, 140, 91 |
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Physical and thermal properties
| Molar volume | 15.18 cm³/mol |
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| Heat of fusion | 15 kJ/mol |
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| Heat of vaporization | 470 kJ/mol |
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| Specific heat | 99.1 J/(kg·K) |
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| Thermal conductivity | 47 W/(m·K) |
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| Thermal expansion | 1.18 × 10⁻⁵ K⁻¹ |
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| Mohs hardness | --- |
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| Young’s modulus | --- |
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| Crystal structure | Centered tetragonal |
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| Magnetic type | Paramagnetic |
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Identification and history
| CAS number | 7440-13-3 |
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| Latin name | Protactinium |
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| Discovered by | K. Fajans, O. H. Göhring · 1913 |
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| Named by | Otto Hahn · 1917 |
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Natural isotopes
| Isotope | Mass (u) | Abundance |
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| 231Pa | 231.035883 | 100 % |
Data checked against PubChem (NIH), IUPAC, the NIST Atomic Spectra Database and the IAEA (NUBASE and AME), the same data you see in the app.
A dash (---) means there is no reliable published measurement. The app never makes one up.
See Protactinium in the app
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Periodic Table