LanthanideElement 69 · f-block

Thulium (Tm)

Thulium (Tm) is a lanthanide with atomic number 69 and a standard atomic weight of 168.93. At room temperature it is a solid that melts at 1,545 °C and boils at 1,950 °C.

69TmThulium168.93

Atomic number

protons

Atomic weight

168.93

CIAAW 2024

Melting point

1,545 °C

1,818.15 K

Boiling point

1,950 °C

2,223.15 K

Density

9.321 g/cm³

near room temperature

Electronegativity

1.25

Pauling scale

State at 25 °C

Solid

Discovered

1879

Sweden

Thulium at a glance

Identity

Symbol	Tm
Atomic number (Z)	69
Atomic weight	168.93 ± 0.01 (CIAAW 2024)
Category	Lanthanide
Group	— (f-block: lanthanide series)
Period	6
Block	f-block
CAS number	7440-30-4

Atomic properties

Electron configuration	[Xe] 4f¹³ 6s² Full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹³ 5s² 5p⁶ 6s²
Electrons per shell	2, 8, 18, 31, 8, 2
Oxidation states	+3 Less common: 0, +1, +2
Electronegativity (Pauling)	1.25
First ionisation energy	6.184 eV (596.7 kJ/mol) 2nd: 12.065 eV · 3rd: 23.66 eV
Electron affinity	1.029 eV
Atomic radius (empirical)	175 pm
Covalent radius	164 pm
Van der Waals radius	227 pm

Physical properties

State at 25 °C	Solid
Melting point	1,545 °C · 2,813 °F · 1,818.15 K
Boiling point	1,950 °C · 3,542 °F · 2,223.15 K
Density	9.321 g/cm³
Specific heat capacity	0.16 J/(g·K)
Crystal structure	Hexagonal

Isotopes & abundance

Natural occurrence	Primordial
Radioactive	No
Natural isotopes (abundance)	¹⁶⁹Tm 100%
Stable isotopes	2
Known isotopes	39
Abundance in Earth's crust	0.52 mg/kg
Abundance in seawater	0.00000017 mg/L

Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.

Thulium electron configuration

Thulium has 69 electrons. Written in short form, its ground-state configuration is [Xe] 4f¹³ 6s² — the electrons of the noble gas xenon, plus 4f¹³ 6s².Written out in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹³ 5s² 5p⁶ 6s².

4f↑↓↑↓↑↓↑↓↑↓↑↓↑

6s↑↓

Discovery and name

Discovered

1879 — Per Teodor Cleve · Sweden.

Name

Thule, the ancient name for a far-northern land (Scandinavia).

Known for

One of the rarest lanthanides; used in portable X-ray sources and some lasers.

Isotopes of thulium

Naturally occurring thulium is a mix of 1 isotope. Each has 69 protons; they differ in the number of neutrons:

Isotope	Protons	Neutrons	Natural abundance	Stability
¹⁶⁹Tm	69	100	100%	stable

39 isotopes of thulium are known in our data set (half-lives as published in nuclear data tables).

Thulium and its neighbours in the lanthanide series

Neighbouring elements in the same f-block series:

Element	Z	Atomic weight	Melting point	Boiling point	Density	Electronegativity
Dysprosium	66	162.50	1,412 °C	2,567 °C	8.55 g/cm³	1.22
Holmium	67	164.93	1,472 °C	2,700 °C	8.8 g/cm³	1.23
Erbium	68	167.26	1,529 °C	2,868 °C	9.07 g/cm³	1.24
Thulium	69	168.93	1,545 °C	1,950 °C	9.321 g/cm³	1.25
Ytterbium	70	173.05	824 °C	1,196 °C	6.9 g/cm³	1.1
Lutetium	71	174.97	1,663 °C	3,402 °C	9.84 g/cm³	1.27

Frequently asked questions

6 answers

What is the atomic mass of thulium?

The standard atomic weight of thulium is 168.93 ± 0.01 (CIAAW 2024), so one mole weighs about 168.93 grams.

How many protons, neutrons and electrons does thulium have?

A neutral thulium atom has 69 protons and 69 electrons. Its most common isotope, thulium-169, has 100 neutrons.

What is the electron configuration of thulium?

[Xe] 4f¹³ 6s² (in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹³ 5s² 5p⁶ 6s²).

What are the melting and boiling points of thulium?

Thulium melts at 1,545 °C (2,813 °F) and boils at 1,950 °C (3,542 °F).

Is thulium a metal?

Thulium is a metal. It is in the f-block, period 6, and is a solid at room temperature.

Who discovered thulium?

1879 — Per Teodor Cleve (Sweden).