What is the electron configuration of thulium?

Thulium (Tm, Z=69) has the ground-state electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹³ 5s² 5p⁶ 6s². Its 69 electrons fill the xenon core (54 electrons) and then add 4f¹³ 6s².

What is the noble-gas shorthand for thulium?

The abbreviated notation is [Xe] 4f¹³ 6s², where [Xe] represents the filled xenon core (1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁶, 54 electrons). The remaining 15 electrons occupy the 4f¹³ and 6s² subshells.

Why does thulium not have an exception like gadolinium?

Gadolinium gains stability by achieving a half-filled 4f⁷ subshell, which maximizes exchange energy among parallel-spin electrons. At thulium, the 4f subshell holds 13 electrons — one short of full. Neither the 4f¹³ nor an alternative 4f¹² 5d¹ arrangement offers a comparable stabilization bonus, so thulium follows standard Aufbau filling.

What oxidation states does thulium form?

Thulium predominantly forms Tm³⁺ by losing both 6s electrons and one 4f electron, leaving the configuration [Xe] 4f¹². The +2 state (Tm²⁺, [Xe] 4f¹³) is accessible in a few compounds and is more stable for thulium than for most other lanthanides because the resulting f¹³ shell is one step away from a fully filled f¹⁴.

How many unpaired electrons does thulium have?

Thulium has 1 unpaired electron in its ground state. The 13 electrons in the 4f subshell fill six orbitals doubly (12 electrons) and leave one orbital with a single electron; the 6s² electrons are paired. So the total unpaired count is 1.

What is thulium used for?

Key applications: (1) Thulium-170, produced by neutron activation, emits 84 keV gamma rays and is used in compact, batteryless portable X-ray sources. (2) Thulium-doped fiber lasers (Tm:fiber) operate near 2 µm (1900–2050 nm), a wavelength absorbed strongly by water, making them useful in laser surgery (urology, ophthalmology) and LIDAR. (3) Thulium is used in high-temperature superconductor research and as a dopant in solid-state lasers.

How does thulium sit in the lanthanide series?

Thulium (Z=69) is the 13th lanthanide (counting from lanthanum Z=57). It sits between erbium (Z=68, [Xe] 4f¹² 6s²) and ytterbium (Z=70, [Xe] 4f¹⁴ 6s²). Ytterbium completes the 4f subshell; thulium has one electron fewer. The lanthanide contraction means thulium's ionic radius is slightly smaller than erbium's.

Thulium Electron Configuration

Name: Thulium Electron Configuration
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Author: Nham Vu

Reference tool for thulium's electron configuration (1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹³ 5s² 5p⁶ 6s²), abbreviated [Xe] 4f¹³ 6s², orbital box diagram, quantum numbers, and fiber-laser and portable X-ray applications.

Z = 69 Tm Thulium

Thulium — Electron Configuration

Atomic number 69 · Lanthanide · Period 6 · f-block · Near-complete 4f¹³

[Xe] 4f¹³ 6s² 69 electrons 1 unpaired 4f electron No Aufbau exception

Near-complete 4f subshell — no exception

With 13 of 14 possible 4f electrons, thulium follows standard Aufbau filling. Unlike gadolinium (which diverts one electron to 5d¹ for a half-filled 4f⁷), thulium gains no measurable exchange-energy advantage from any alternative arrangement. The result is the straightforward [Xe] 4f¹³ 6s² — one step below the fully-filled ytterbium ([Xe] 4f¹⁴ 6s²).

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 (near-complete — 13 of 14)	13	14	4f¹³
6s	s orbital, shell n=6 (outermost)	2	2	6s²
Total		69

Full Configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹³ 5s² 5p⁶ 6s²

All subshells written explicitly; 69 electrons total.

Noble-Gas Shorthand

[Xe] 4f¹³ 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 4f¹³ 31 / 32 electrons (97%)

Shell 5 (n=5) — 5s² 5p⁶ 8 / 50 electrons (16%)

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

Shell 4 holds 31 of its 32-electron maximum (4f stops at 13; the 4f¹⁴ completes in ytterbium, Z=70). Shell 5 has only 8 electrons — the 5s² and 5p⁶ subshells from the xenon core, with no 5d electrons in thulium.

The Xenon Core — [Xe]

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

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

The remaining 15 electrons — 4f¹³ 6s² — govern thulium's chemistry, magnetic properties, and oxidation states.

Orbital Box Diagram — Valence Region

The xenon core (1s² through 5p⁶, 54 electrons) is shown in compact form. The valence subshells 4f 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

13 electrons — 6 orbitals paired + 1 orbital with single electron (1 unpaired)

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 subshell.

Thulium has 1 unpaired electron in its ground state (the lone electron in the seventh 4f orbital). The 6s² electrons are paired. This contrasts sharply with gadolinium (8 unpaired) and gives thulium a much smaller magnetic moment.

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	6	0	0	+½	6s
69	6	0	0	−½	6s

Electrons 1–54 fill the xenon core (not shown). n = principal quantum number; l = azimuthal (3→f, 0→s); mₗ = magnetic (−3 to +3 for f); mₛ = spin. Electrons 55–61 fill all seven 4f orbitals spin-up; electrons 62–67 pair back into the first six orbitals, leaving electron 67 in mₗ = +2 and the seventh orbital (mₗ = +3) with its spin-up electron unpaired.

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, electrons sequentially fill the 4f subshell; thulium reaches 4f¹³ — one electron short of the fully filled 4f¹⁴.
No half-filled or fully-filled exception applies at thulium: neither 4f¹³ nor the alternative 4f¹² 5d¹ offers a measurable exchange-energy advantage.
The 6s subshell fills before the 4f in the free atom due to its lower energy at that point in the sequence, giving the outer shell 6s².
Noble-gas notation replaces the 54-electron xenon core with [Xe], leaving the condensed configuration [Xe] 4f¹³ 6s².
Hund's rule fills the 13 electrons across seven 4f orbitals: six orbitals are doubly occupied (12 electrons) and one holds a single electron, yielding 1 unpaired electron in the 4f shell plus the paired 6s² electrons.

Use cases

Quick reference for chemistry homework or exam review on lanthanide electron configurations.
Understand why thulium does not show an exception like gadolinium — 4f¹³ offers no special exchange-energy stabilization.
Visualize how the 4f subshell fills across the late lanthanides (Tb → Yb) and contrast with the Gd exception.
Teaching aid for noble-gas notation, Hund's rule, and near-complete f-subshell occupancy.
Background for understanding why thulium-170 (Tm-170) is used in portable X-ray generators.
Compare thulium to adjacent lanthanides (erbium Z=68, ytterbium Z=70) to observe 4f-electron trends.

Frequently Asked Questions

Last updated: 2026-07-25 · Reviewed by Nham Vu