What is the electron configuration of protactinium?

Protactinium (Pa, Z=91) has the ground-state configuration [Rn] 5f² 6d¹ 7s², or in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 6s² 6p⁶ 5f² 6d¹ 7s².

Why does protactinium have 6d¹ instead of the expected 5f³?

At Z=91 the 5f and 6d orbitals are nearly identical in energy. Placing one electron in 6d while filling two in 5f (5f² 6d¹ 7s²) provides lower total energy through enhanced exchange interactions between unpaired electrons in different subshells compared to 5f³ 7s².

How many valence electrons does protactinium have?

Protactinium has 5 valence electrons: two in 5f, one in 6d, and two in 7s. It commonly loses all five to form Pa⁵⁺, or four (losing 6d and 7s) to form Pa⁴⁺.

What block is protactinium in?

Protactinium is formally an f-block element and is the second member of the actinide series. However, because it retains a 6d electron, some classifications note it has mixed d/f character.

Is protactinium radioactive?

Yes. All isotopes of protactinium are radioactive. The most stable isotope is Pa-231 with a half-life of about 32,760 years. Pa-234m (t½ = 1.17 min) is a short-lived decay product of U-238.

Protactinium Electron Configuration

Name: Protactinium Electron Configuration
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Author: Nham Vu

Reference for protactinium's electron configuration: [Rn] 5f² 6d¹ 7s² (full: 1s²…6p⁶ 5f² 6d¹ 7s²). Includes orbital filling table, shell breakdown, and anomalous 5f+6d filling explanation.

Z = 91 Pa Protactinium

Protactinium — Electron Configuration

Atomic number 91 · Actinide series · Period 7 · f-block (with 6d anomaly)

[Rn] 5f² 6d¹ 7s² 91 electrons 5 valence e⁻ Anomalous 5f²6d¹

Why 5f² 6d¹ and Not 5f³?

Strict Aufbau order predicts the 89th–91st electrons would all enter 5f, giving [Rn] 5f³ 7s². In practice, at Z=91 the 5f and 6d sub-levels are nearly degenerate in energy. Placing one electron in 6d while two occupy 5f — configuration [Rn] 5f² 6d¹ 7s² — lowers total energy through favorable spin-exchange interactions between electrons in different but close-lying subshells. This behavior is common for early actinides (Th through Am); the 5f-6d crossover solidifies further along the actinide row where 5f drops well below 6d.

Noble-Gas Shorthand

[Rn] 5f² 6d¹ 7s²

[Rn] = 86-electron radon core.

Full Expanded Configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 6s² 6p⁶ 5f² 6d¹ 7s²

All 91 electrons written by subshell.

Subshell Breakdown

Subshell	Type	Electrons	Capacity	Notation
1s	s orbital, shell n=1	2	2	1s²
2s	s orbital, shell n=2	2	2	2s²
2p	p orbitals, shell n=2	6	6	2p⁶
3s	s orbital, shell n=3	2	2	3s²
3p	p orbitals, shell n=3	6	6	3p⁶
3d	d orbitals, shell n=3	10	10	3d¹⁰
4s	s orbital, shell n=4	2	2	4s²
4p	p orbitals, shell n=4	6	6	4p⁶
4d	d orbitals, shell n=4	10	10	4d¹⁰
4f	f orbitals, shell n=4	14	14	4f¹⁴
5s	s orbital, shell n=5	2	2	5s²
5p	p orbitals, shell n=5	6	6	5p⁶
5d	d orbitals, shell n=5	10	10	5d¹⁰
6s	s orbital, shell n=6	2	2	6s²
6p	p orbitals, shell n=6	6	6	6p⁶
5f	f orbitals, shell n=5	2	14	5f²
6d	d orbitals, shell n=6	1	10	6d¹
7s	s orbital, shell n=7	2	2	7s²
Total		91

Valence subshells (5f, 6d, and 7s) are highlighted. Note: 5f appears after 6p in the filling order, before 6d, due to energy-level crossings in heavy elements.

Shell Fill Summary

Shell 1 (n=1) — 1s² 2 / 2 electrons (100%)

Shell 2 (n=2) — 2s² 2p⁶ 8 / 8 electrons (100%)

Shell 3 (n=3) — 3s² 3p⁶ 3d¹⁰ 18 / 18 electrons (100%)

Shell 4 (n=4) — 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 32 / 32 electrons (100%)

Shell 5 (n=5) — 5s² 5p⁶ 5d¹⁰ 5f² 20 / 50 electrons (40%)

Shell 6 (n=6) — 6s² 6p⁶ 6d¹ 9 / 32 electrons (28%)

Shell 7 (n=7) — 7s² 2 / 32 electrons (6%)

Valence Electron Quantum Numbers

Electron	n (principal)	l (azimuthal)	Sublevel	m_l range	m_s
5f (e1)	5	3	f orbital	-3 to +3	+½
5f (e2)	5	3	f orbital	-3 to +3	+½ (different m_l)
6d (e1)	6	2	d orbital	-2 to +2	+½
7s (e1)	7	0	s orbital	0	+½
7s (e2)	7	0	s orbital	0	-½ (paired)

Summary

Reference for protactinium's electron configuration: [Rn] 5f² 6d¹ 7s² (full: 1s²…6p⁶ 5f² 6d¹ 7s²). Includes orbital filling table, shell breakdown, and anomalous 5f+6d filling explanation.

How it works

Protactinium's 91 electrons fill orbitals in order of increasing energy following the Aufbau principle, with exceptions.
Radon ([Rn], Z=86) provides the filled core: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 6s² 6p⁶.
The next five electrons in Pa fill as 5f² 6d¹ 7s² rather than the strict Aufbau 5f³ 7s².
At Z=91 the 5f and 6d sub-levels are extremely close in energy; one 6d electron is retained to maximize spin-exchange stabilization.
Valence electrons are 5f² 6d¹ 7s², giving Pa common oxidation states of +4 and +5.
The +5 state (losing 5f² 6d¹ 7s² all five outer electrons) is the most stable oxidation state for protactinium.

Use cases

Chemistry reference for students studying actinide electron configurations.
Compare Pa's anomalous 5f+6d filling against strict Aufbau expectations.
Understand why early actinides retain 6d electrons while later actinides do not.
Verify quantum numbers and subshell counts for exam preparation.
Teaching aid for 5f vs 6d energy competition in heavy elements.

Frequently Asked Questions

Last updated: 2026-07-24 · Reviewed by Nham Vu