Protactinium (Pa): electron configuration, atomic mass and facts
Protactinium (Pa) is element 91 of the periodic table, an actinide in period 7 (f-block). Its electron configuration is [Rn] 5f2 6d1 7s2, with electrons arranged in shells 2, 8, 18, 32, 20, 9, 2, and its standard atomic weight is 231.04 u.
▶ Watch Protactinium’s electrons orbit in the interactive periodic tableProtactinium at a glance
| 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 | 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 5f2 6s2 6p6 6d1 7s2 |
| Noble-gas shorthand | [Rn] 5f2 6d1 7s2 |
| Electrons per shell | 2, 8, 18, 32, 20, 9, 2 |
| Electronegativity (Pauling) | 1.5 |
| State at 20 °C | solid |
Where is Protactinium on the periodic table?
Protactinium is in period 7 of the f-block, the actinide row printed below the main table. It has 7 electron shells, and this row is where the 5f subshell fills (its own configuration is an exception, explained below).
How to work out the electron configuration of Protactinium
Put 91 electrons into subshells in order of rising energy, filling each before starting the next: 1s² (2), 2s² (4), 2p⁶ (10), 3s² (12), 3p⁶ (18), 4s² (20), 3d¹⁰ (30), 4p⁶ (36), 5s² (38), 4d¹⁰ (48), 5p⁶ (54), 6s² (56), 4f¹⁴ (70), 5d¹⁰ (80), 6p⁶ (86), 7s² (88), 5f³ (91). Protactinium is one of the exceptions to this filling order: experiments show its ground state is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f² 6d¹ rather than 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f³, because the neighbouring subshells are so close in energy. In shorthand, [Rn] stands for the 86 electrons of radon, giving [Rn] 5f² 6d¹ 7s².
How electronegative is Protactinium?
Protactinium has a Pauling electronegativity of 1.5, ranking 55 of the 100 elements with a known value (fluorine is highest at 3.98). That is in the middle of the range.
Frequently asked questions
What is the electron configuration of Protactinium?
The electron configuration of Protactinium (Pa) is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 5f2 6s2 6p6 6d1 7s2. In noble-gas shorthand it is written [Rn] 5f2 6d1 7s2.
How many electrons does Protactinium have in each shell?
Protactinium has 91 electrons arranged in shells as 2, 8, 18, 32, 20, 9, 2.
What is the atomic mass of Protactinium?
The standard atomic weight of Protactinium is 231.04 u, so one mole of Protactinium atoms has a mass of about 231.04 g.
Is Protactinium a metal?
Yes. Protactinium is an actinide.
Explore nearby elements
More actinides: Actinium, Thorium, Uranium, Neptunium, Plutonium, Americium. See all elements on the periodic table.