Why does lutetium have a 5d¹ electron?

The 4f subshell holds a maximum of 14 electrons. In lutetium (Z=71), all 14 4f slots are filled by electrons 55–68 (after the xenon core). The 69th and 70th electrons pair in 6s, and the 71st electron has no room in 4f, so it goes to 5d¹. This is not an anomaly but the expected consequence of 4f being completely full.

What is the noble-gas shorthand for lutetium?

The abbreviated notation is [Xe] 4f¹⁴ 5d¹ 6s², where [Xe] represents the filled xenon core (1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁶, 54 electrons).

How many unpaired electrons does lutetium have?

Lutetium has one unpaired electron in its ground state — the single electron in the 5d¹ orbital. All 14 electrons in the 4f subshell are paired (7 pairs across 7 orbitals), and the 6s² electrons are paired. Lutetium is therefore weakly paramagnetic.

Is lutetium a lanthanide or a transition metal?

This is a classification debate. IUPAC traditionally lists lutetium as the last lanthanide (Period 6, f-block). However, because its ground state has a filled 4f¹⁴ subshell and one 5d electron — similar to scandium and yttrium — some chemists classify it as a Group 3 d-block transition metal. Both views appear in authoritative textbooks.

What is lutetium used for?

Key applications: (1) Lutetium oxyorthosilicate (Lu₂SiO₅:Ce, LSO) is a bright, fast scintillator in PET scanners. (2) Lu-177, produced in nuclear reactors, is the active isotope in targeted radioligand therapies (e.g., Lutathera for neuroendocrine tumors, PSMA-617 for prostate cancer). (3) Lutetium aluminum garnet (LuAG) is used in LED phosphors and laser host crystals. (4) Lutetium oxide improves high-refractive-index glass and serves as a catalyst in organic synthesis.

What oxidation state does lutetium form?

Lutetium forms exclusively the +3 oxidation state (Lu³⁺) under all normal chemical conditions. It loses 5d¹ and 6s² electrons, exposing the stable [Xe] 4f¹⁴ core. Unlike earlier lanthanides (Eu, Sm, Yb), lutetium has no accessible +2 or +4 state because the completely filled 4f¹⁴ and the empty 5d each represent a stable boundary with no intermediate stability.

Lutetium Electron Configuration

Name: Lutetium Electron Configuration
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Author: Nham Vu

Reference tool for lutetium's electron configuration ([Xe] 4f¹⁴ 5d¹ 6s²), orbital diagram, why the full 4f¹⁴ subshell forces a 5d¹ electron, and key properties including Lu-177 cancer therapy.

Z = 71 Lu Lutetium

Lutetium — Electron Configuration

Atomic number 71 · Last lanthanide · Period 6 · f-block · Full 4f¹⁴ subshell

[Xe] 4f¹⁴ 5d¹ 6s² 71 electrons 1 unpaired (5d¹) Last lanthanide

Why lutetium has a 5d¹ electron

Lutetium's 4f subshell is completely filled (4f¹⁴ — all 14 slots occupied). With no room left in 4f, the next electron must enter 5d. This is not an anomaly like gadolinium's half-filled 4f⁷ exception — it is the expected result of 4f reaching capacity. Lutetium's [Xe] 4f¹⁴ 5d¹ 6s² configuration marks the end of the lanthanide series.

Subshell Breakdown

Subshell	Type	Electrons	Max 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 (part of Xe core)	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 (part of Xe core)	10	10	4d¹⁰
5s	s orbital, shell n=5	2	2	5s²
5p	p orbitals, shell n=5 (completes Xe core)	6	6	5p⁶
4f	f orbitals, shell n=4 (fully filled)	14	14	4f¹⁴
5d	d orbitals, shell n=5 (1 electron — 4f full)	1	10	5d¹
6s	s orbital, shell n=6 (outermost)	2	2	6s²
Total		71

Full Configuration

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

All subshells written explicitly; 71 electrons total.

Noble-Gas Shorthand

[Xe] 4f¹⁴ 5d¹ 6s²

[Xe] = xenon core, 54 electrons (1s² through 5p⁶).

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 5d¹ 9 / 50 electrons (18%)

Shell 6 (n=6) — 6s² 2 / 72 electrons (3%)

Shell 4 is completely filled (4s² 4p⁶ 4d¹⁰ 4f¹⁴ = 32 electrons). Shell 5 holds 9 of its 50-electron maximum — the 5d subshell receives just one electron. Shell 6 carries only the 6s² pair.

The Xenon Core — [Xe]

The first 54 electrons of lutetium match the complete electron configuration of xenon (Z=54):

[Xe] = 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁶

The remaining 17 electrons — 4f¹⁴ 5d¹ 6s² — govern lutetium's chemistry and oxidation state. Removing the 5d¹ and 6s² electrons gives Lu³⁺, leaving the inert [Xe] 4f¹⁴ core.

Orbital Box Diagram — Valence Region

The xenon core (1s² through 5p⁶, 54 electrons) is shown in compact form. The valence subshells 4f, 5d, and 6s are shown in detail.

[Xe]

core: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁶

54 electrons, all paired — xenon configuration

14 electrons — all 7 orbitals fully paired (complete 4f¹⁴)

1 unpaired electron in first 5d orbital; orbitals 2–5 empty

2 electrons (paired) — outermost shell

Each box = one orbital. Opposing arrows = paired electrons (+½ and −½ spin). Single up arrow = unpaired electron. Colored border = partially filled or active subshell.

Lutetium has 1 unpaired electron (5d¹). The 4f¹⁴ subshell is completely paired. Lutetium is weakly paramagnetic compared to earlier lanthanides like gadolinium (8 unpaired) or terbium (6 unpaired).

Quantum Numbers — Valence Electrons (post-[Xe])

Electron #	n	l	mₗ	mₛ	Subshell
55	4	3	-3	+½	4f
56	4	3	-2	+½	4f
57	4	3	-1	+½	4f
58	4	3	0	+½	4f
59	4	3	1	+½	4f
60	4	3	2	+½	4f
61	4	3	3	+½	4f
62	4	3	-3	−½	4f
63	4	3	-2	−½	4f
64	4	3	-1	−½	4f
65	4	3	0	−½	4f
66	4	3	1	−½	4f
67	4	3	2	−½	4f
68	4	3	3	−½	4f
69	5	2	-2	+½	5d
70	6	0	0	+½	6s
71	6	0	0	−½	6s

Electrons 1–54 fill the xenon core (not shown). n = principal quantum number; l = azimuthal (3→f, 2→d); mₗ = magnetic (−3 to +3 for f, −2 to +2 for d); mₛ = spin. The 14 electrons in 4f are arranged across all seven mₗ values, each doubly occupied. Electron 69 sits in 5d; electrons 70–71 pair in 6s.

Identity

SymbolLu Atomic number71 Atomic mass174.967 u SeriesLanthanide (last) Period6 Blockf-block / d-block (debated) Discovery1907 (Urbain, von Welsbach)

Physical Properties

State (25 °C)Solid Melting point1663 °C Boiling point3402 °C Density9.84 g/cm³ ColorSilvery white Crystal structureHCP

Electronic Properties

Valence electrons17 (4f¹⁴ 5d¹ 6s²) Unpaired electrons1 (5d¹) Common oxidation state+3 (only practical) Electronegativity1.27 (Pauling) 1st ionization energy523.5 kJ/mol Magnetic behaviorWeakly paramagnetic

Applications

PET scan scintillators — lutetium oxyorthosilicate (Lu₂SiO₅:Ce, LSO) is a bright, fast crystal widely used in modern PET detector rings.
Lu-177 targeted radiotherapy — reactor-produced Lu-177 is the active isotope in Lutathera (neuroendocrine tumors) and Pluvicto/PSMA-617 (prostate cancer).
LED phosphors and laser hosts — lutetium aluminum garnet (LuAG) and lutetium oxide phosphors are used in high-efficiency white LEDs and solid-state lasers.
High-refractive-index glass and catalysts — lutetium oxide (Lu₂O₃) raises refractive index in specialty optical glass and acts as a catalyst in organic synthesis.

Summary

How it works

The Aufbau principle fills orbitals from lowest to highest energy, placing the xenon core (1s² through 5p⁶, 54 electrons) first.
After the xenon core, lanthanide filling continues: 4f orbitals accommodate electrons 55 through 68, completing the 4f¹⁴ subshell.
With 4f fully occupied (14 electrons across seven orbitals, each paired), no more electrons can enter 4f.
Electron 69 and 70 enter the 5s and 5p already covered in [Xe]; the 71st electron goes to 5d¹ as the next available subshell.
The resulting configuration [Xe] 4f¹⁴ 5d¹ 6s² places lutetium in Group 3 of period 6 — its 5d¹ 6s² valence mirrors lanthanum and makes it a transition-metal-like element in some classifications.
Lutetium loses its 5d¹ and 6s² electrons to form the stable Lu³⁺ ion, leaving the inert [Xe] 4f¹⁴ core.

Use cases

Quick reference for chemistry homework or exam review on lanthanide electron configurations.
Understand why lutetium is the last lanthanide — once 4f¹⁴ is complete, the series ends.
Visualize how 4f, 5d, and 6s subshells relate at the end of the lanthanide row.
Explain lutetium's role in PET scan detectors and Lu-177 targeted cancer therapy.
Compare lutetium to gadolinium (half-filled 4f exception) and lanthanum (first lanthanide) to see 4f-filling trends.
Teaching aid for d-block vs. f-block boundaries and Group 3 element classification debates.
Understand lutetium's exclusive +3 oxidation state from its valence electron configuration.

Frequently Asked Questions

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