What is the electron configuration of neptunium?

Neptunium (Np, Z=93) has the ground-state configuration [Rn] 5f⁴ 6d¹ 7s², or in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 6s² 6p⁶ 5f⁴ 6d¹ 7s².

Why does neptunium have 6d¹ instead of a full 5f⁵?

At Z=93 the 5f and 6d sublevels are nearly degenerate. Relativistic contraction and electron-electron repulsion in the half-forming 5f shell make placing one electron into 6d energetically favorable over filling 5f to five electrons. This anomaly is shared by several early actinides.

How many valence electrons does neptunium have?

Neptunium has 7 electrons beyond the [Rn] core (5f⁴ + 6d¹ + 7s² = 7), but the chemically accessible valence electrons are the 5f, 6d, and 7s electrons. Common oxidation states are +4 and +5; it can reach +7 in strongly oxidizing conditions.

What block is neptunium in on the periodic table?

Neptunium is an f-block element and a member of the actinide series, placed in period 7. Despite having a 6d¹ electron, its differentiating electrons are primarily 5f, so it is classified in the f-block.

What is the noble-gas shorthand for neptunium?

[Rn] 5f⁴ 6d¹ 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 neptunium naturally occurring?

Neptunium occurs in trace amounts in uranium ores via neutron capture reactions on U-238, but it is predominantly synthetic. Np-237 (half-life 2.14 million years) is the most significant isotope and accumulates in spent nuclear fuel.

Neptunium Electron Configuration

Name: Neptunium Electron Configuration
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Neptunium (Np, Z=93) ground-state configuration is [Rn] 5f⁴ 6d¹ 7s² — an anomalous case where one electron sits in 6d rather than filling 5f sequentially.

Z = 93 Np Neptunium

Neptunium — Electron Configuration

Atomic number 93 · Actinide series · Period 7 · f-block (anomalous 6d¹)

[Rn] 5f⁴ 6d¹ 7s² 93 electrons 7 outer e⁻ Anomalous 6d¹

Why 5f⁴ 6d¹ and Not 5f⁵?

Strict Aufbau order predicts the 93rd electron would continue filling 5f, giving [Rn] 5f⁵ 7s². In practice, the 5f and 6d sublevels are nearly degenerate in early actinides. At Z=93, interelectronic repulsion within the growing 5f shell — combined with relativistic effects — makes placing one electron into 6d slightly lower in energy than filling 5f to five electrons. Neptunium therefore adopts [Rn] 5f⁴ 6d¹ 7s². This pattern appears in several early actinides: Pa (5f² 6d¹ 7s²), U (5f³ 6d¹ 7s²), and Np (5f⁴ 6d¹ 7s²). From plutonium onward, 5f filling resumes without 6d participation.

Noble-Gas Shorthand

[Rn] 5f⁴ 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 5f⁴ 6d¹ 7s²

All 93 electrons written by subshell.

Subshell Breakdown

Subshell	Type	Electrons	Capacity	Notation
1s	s orbital	2	2	1s²
2s	s orbital	2	2	2s²
2p	p orbitals	6	6	2p⁶
3s	s orbital	2	2	3s²
3p	p orbitals	6	6	3p⁶
3d	d orbitals	10	10	3d¹⁰
4s	s orbital	2	2	4s²
4p	p orbitals	6	6	4p⁶
4d	d orbitals	10	10	4d¹⁰
4f	f orbitals	14	14	4f¹⁴
5s	s orbital	2	2	5s²
5p	p orbitals	6	6	5p⁶
5d	d orbitals	10	10	5d¹⁰
5f	f orbitals	4	14	5f⁴
6s	s orbital	2	2	6s²
6p	p orbitals	6	6	6p⁶
6d	d orbitals	1	10	6d¹
7s	s orbital	2	2	7s²
Total		93

Valence subshells (5f, 6d, 7s) are highlighted in primary color. All others form the [Rn] core.

Shell Fill Summary

Shell 1 (n=1) — 1s² 2 / 2 e⁻ (100%)

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

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

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

Shell 5 (n=5) — 5s² 5p⁶ 5d¹⁰ 5f⁴ 22 / 50 e⁻ (44%)

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

Shell 7 (n=7) — 7s² 2 / 32 e⁻ (6%)

Quantum Numbers for Outer Electrons

Each electron is described by four quantum numbers: n (principal), ℓ (azimuthal), m_ℓ (magnetic), m_s (spin). The table below covers neptunium's valence electrons in the 5f, 6d, and 7s subshells.

Subshell	n	ℓ	Possible m_ℓ	Electrons	m_s
5f	5	3	−3, −2, −1, 0, +1, +2, +3	4	+½ for each (Hund's rule: singly filled first)
6d	6	2	−2, −1, 0, +1, +2	1	+½
7s	7	0	0	2	+½ and −½

Subshell ℓ values: s=0, p=1, d=2, f=3. Hund's rule applies: electrons fill separate m_ℓ orbitals with parallel spin before pairing.

Unpaired Electrons & Magnetic Character

Neptunium has 5 unpaired electrons in the ground state: four in 5f (each singly occupied by Hund's rule) and one in 6d. This gives neptunium a large magnetic moment and strongly paramagnetic behavior. The 7s electrons are paired and contribute no net spin.

Summary

Neptunium (Np, Z=93) ground-state configuration is [Rn] 5f⁴ 6d¹ 7s² — an anomalous case where one electron sits in 6d rather than filling 5f sequentially.

How it works

Neptunium has 93 electrons that fill atomic orbitals following the Aufbau principle.
Radon ([Rn], Z=86) provides the filled 86-electron core: 1s² through 6p⁶.
After the [Rn] core, Aufbau predicts filling: 5f¹ (Th anomaly), continuing to 5f⁵ for Z=93.
In practice, at Z=93 the 5f and 6d sublevels are nearly degenerate; one electron occupies 6d¹, yielding [Rn] 5f⁴ 6d¹ 7s².
This mirrors a pattern seen in several early actinides (Pa, U, Np) where partial 6d occupancy competes with 5f filling.
The three outer electrons (5f⁴, 6d¹, 7s²) make neptunium chemically versatile, showing oxidation states from +3 to +7.

Use cases

Chemistry reference for students studying actinide or heavy-element electron configurations.
Understand the 5f vs 6d energy competition that defines early actinide behavior.
Verify quantum numbers and subshell counts for exam preparation.
Compare neptunium to neighboring actinides (uranium, plutonium) to trace the 5f filling trend.
Teaching aid for relativistic effects, electron shielding, and anomalous configurations.
Nuclear chemistry context: neptunium-237 is the longest-lived Np isotope and a key byproduct in nuclear reactors.

Frequently Asked Questions

Last updated: 2026-07-23 · Reviewed by Nham Vu