What is the electron configuration of thorium?

Thorium (Th, Z=90) has the ground-state configuration [Rn] 6d² 7s², or in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 6s² 6p⁶ 6d² 7s².

Why does thorium have 6d² instead of 5f²?

At Z=90, the 6d and 5f sublevels are nearly equal in energy. Relativistic effects and electron-electron repulsion favor placing both outer electrons in separate 6d orbitals (obeying Hund's rule) over pairing in 5f. The result is [Rn] 6d² 7s². From Pa (Z=91) onward, 5f becomes clearly lower and begins filling.

How many valence electrons does thorium have?

Thorium has 4 valence electrons: two in 6d and two in 7s. It most commonly loses all four to form Th⁴⁺, though Th²⁺ and Th³⁺ are known in some compounds.

What block does thorium belong to?

Thorium is conventionally placed in the f-block (actinide series), but its actual ground-state electron configuration puts the differentiating electrons in 6d, making it behave like a d-block element. This is why Th chemistry resembles that of early transition metals.

What is the noble-gas shorthand for thorium?

[Rn] 6d² 7s², where [Rn] represents the 86-electron radon core: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 6s² 6p⁶.

Is thorium radioactive?

Yes. All isotopes of thorium are radioactive. Thorium-232 is the most abundant and stable, with a half-life of 14.05 billion years — comparable to the age of the universe. It is a fertile material used in experimental nuclear reactors.

What are the quantum numbers for the last electron of thorium?

The last electron (90th) goes into 6d: principal quantum number n=6, angular momentum l=2, magnetic quantum number mₗ=+1 (the second 6d orbital), and spin mₛ=+½ (unpaired, following Hund's rule).

Thorium Electron Configuration

Name: Thorium Electron Configuration
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Author: Nham Vu

Reference for thorium's electron configuration: [Rn] 6d² 7s² (full: 1s²…6p⁶ 6d² 7s²). Includes orbital filling diagram, shell breakdown, and key element facts.

Z = 90 Th Thorium

Thorium — Electron Configuration

Atomic number 90 · Actinide series · Period 7 · d-block (anomalous)

[Rn] 6d² 7s² 90 electrons 4 valence e⁻ Anomalous 6d²

Why 6d² and Not 5f²?

Strict Aufbau order predicts thorium's two outer electrons would enter 5f, giving [Rn] 5f² 7s². In practice, at Z=90 the 6d and 5f sublevels are nearly equal in energy. Relativistic effects and inter-electron repulsion favor placing both electrons in separate 6d orbitals (spin-parallel, per Hund's rule) over crowding into 5f. Thorium therefore adopts [Rn] 6d² 7s². This mirrors the behavior of hafnium one period above. From protactinium (Z=91) onward, 5f drops below 6d and the true actinide 5f series begins.

Noble-Gas Shorthand

[Rn] 6d² 7s²

[Rn] = 86-electron radon core.

Full Expanded Configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 6s² 6p⁶ 6d² 7s²

All 90 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
6d	d orbitals, shell n=6	2	10	6d²
7s	s orbital, shell n=7	2	2	7s²
Total		90

Valence subshells (6d and 7s) are highlighted in primary color. All others form the [Rn] core.

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 18 / 32 electrons (56%)

Shell 6 (n=6) — 6s² 6p⁶ 6d² 10 / 32 electrons (31%)

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

Quantum Numbers for Valence Electrons

Electron	Subshell	n	l	m_l	m_s
89th (6d, first)	6d¹	6	2 (d)	-2	+½
90th (6d, second)	6d¹	6	2 (d)	-1	+½
87th (7s, first)	7s¹	7	0 (s)	0	+½
88th (7s, second)	7s²	7	0 (s)	0	-½

n = principal, l = angular momentum (0=s,1=p,2=d,3=f), m_l = magnetic, m_s = spin. The two 6d electrons obey Hund's rule — each occupies a distinct m_l value with parallel spin (+½).

Interpreting the Quantum Numbers

n=6, l=2 means the 6d subshell — sixth principal shell, d-type orbitals (l=2).
The d subshell has five orbitals: m_l = −2, −1, 0, +1, +2. Thorium's two 6d electrons occupy m_l=−2 and m_l=−1 with parallel spins (+½ each) per Hund's rule.
n=7, l=0 means the 7s subshell. The single orbital (m_l=0) holds two paired electrons (+½ and −½).
No two electrons share all four identical quantum numbers (Pauli exclusion principle is satisfied).

Summary

Reference for thorium's electron configuration: [Rn] 6d² 7s² (full: 1s²…6p⁶ 6d² 7s²). Includes orbital filling diagram, shell breakdown, and key element facts.

How it works

Thorium's 90 electrons fill orbitals in order of increasing energy following the Aufbau principle.
Radon ([Rn], Z=86) provides the filled 86-electron core: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 6s² 6p⁶.
After the [Rn] core, Aufbau predicts electrons entering 5f, but at Z=90 the 6d orbital is very close in energy to 5f.
Thorium places both outer electrons in 6d, giving the anomalous configuration [Rn] 6d² 7s² instead of the expected [Rn] 5f² 7s².
From protactinium (Z=91) onward, 5f begins filling in earnest — Th sits at the boundary between d-block and f-block character.
Valence electrons are 6d² 7s², giving thorium a common +4 oxidation state (all four outer electrons removed).

Use cases

Chemistry reference for students studying actinide or heavy-element electron configurations.
Compare thorium's anomalous 6d² filling with standard Aufbau 5f predictions.
Understand why thorium is classified as both an actinide and a d-block element.
Verify quantum numbers and subshell counts for exam preparation.
Teaching tool for periodic trends, electron shielding, and energy-level crossings near the 5f-6d boundary.
Nuclear chemistry context: thorium-232 is the basis for the thorium fuel cycle.

Frequently Asked Questions

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