Protactinium (Pa)
Protactinium is element 91, a actinide in period 7 of the periodic table. At room temperature it is a solid.
Electron configuration [Rn] 5f2 6d1 7s2
Electrons per shell 2, 8, 18, 32, 20, 9, 2
Protactinium data
| Atomic number | 91 |
|---|---|
| Symbol | Pa |
| Atomic mass | 231.036 u |
| Category | Actinide |
| Group | — (f-block) |
| Period | 7 |
| Block | f |
| State at 25 °C | solid |
| Melting point | 1845 K (1571.9 °C) |
| Boiling point | 4273 K (3999.9 °C) |
| Density | 15.37 g/cm³ |
| Electronegativity (Pauling) | 1.5 |
| Atomic radius | — |
| 1st ionization energy | 568 kJ/mol |
| Electron affinity | — |
| Oxidation states | +3, +4, +5 |
| Common ion charge | 5+ |
| Discovered | 1913 |
Orbital diagram
Each box is one orbital holding up to two electrons of opposite spin. Following Hund’s rule, electrons spread out singly across a subshell before pairing.
Work through protactinium’s configuration and its ions →
Where protactinium sits
Protactinium belongs to the actinides. Its Pauling electronegativity of 1.5 is mid-range; see the electronegativity chart and periodic trends for context. To use its mass in a calculation, try the molar mass calculator.
Protactinium FAQ
What is the electron configuration of protactinium?
Protactinium (Z = 91) has the configuration 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 4f14 5d10 6s2 6p6 5f2 6d1 7s2. In noble-gas shorthand that is [Rn] 5f2 6d1 7s2. This is an exception to the simple Aufbau prediction (1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 5f3 6s2 6p6 7s2); a half-filled or filled subshell lowers the energy.
How many valence electrons does protactinium have?
3 (2 outer s + 1 d electrons; 2 in the outermost shell).
What is the atomic mass of protactinium?
The standard atomic weight of protactinium is 231.036 u, so one mole weighs 231.036 g.
Is protactinium a metal?
Yes. Protactinium is a actinide.
What is the charge of a protactinium ion?
Protactinium most commonly forms Pa5+.