Electron Configuration of Protactinium (Pa)
Protactinium (Pa, Z = 91) has the electron configuration [Rn] 5f² 6d¹ 7s², filling 7 shells with 2, 8, 18, 32, 20, 9, 2 electrons.
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Protactinium: every subshell, one by one
The table below unpacks [Rn] 5f² 6d¹ 7s² line by line, so you can watch Protactinium's 91 electrons take their seats in order of increasing energy.
| Subshell | Electrons | Total so far |
|---|---|---|
| [Rn] | 0 | 86 |
| 5f | 2 | 88 |
| 6d | 1 | 89 |
| 7s | 2 | 91 |
Electrons per shell (K, L, M…)
Grouped by principal quantum number instead of subshell, Protactinium carries 2 in shell 1, 8 in shell 2, 18 in shell 3, 32 in shell 4, 20 in shell 5, 9 in shell 6, 2 in shell 7. The outermost shell holds 2 electrons — the ones that do all the bonding.
How to derive it yourself
Follow the aufbau order (1s → 2s → 2p → 3s → 3p → 4s → 3d → …), pouring 91 electrons into orbitals of capacity 2 (s), 6 (p), 10 (d) and 14 (f) until none remain. Write the filled inner shells as the nearest noble-gas core — for Protactinium that gives [Rn] 5f² 6d¹ 7s².
Note: Protactinium is one of the textbook exceptions to strict aufbau filling — a half-filled or fully-filled d subshell buys extra stability, so its ground-state configuration is [Rn] 5f² 6d¹ 7s².
Students also ask
How many electron shells does Protactinium have?
Protactinium is in period 7, so its electrons occupy 7 shells, holding 2, 8, 18, 32, 20, 9, 2 electrons respectively.
How many valence electrons does Protactinium have?
Its outermost shell holds 2 electrons, so Protactinium has 2 valence electrons that take part in bonding.
What is the noble-gas shorthand for Protactinium?
The shorthand form is exactly [Rn] 5f² 6d¹ 7s².