Thulium (Tm)
Element 69 · Lanthanide · atomic mass 168.93 u · f-block, period 6
Electron shells: 2, 8, 18, 31, 8, 2
Thulium is the second rarest of the lanthanides and the least commercially used. Its radioactive isotope powers small portable X-ray sources.
A thulium-170 source lets field medics take X-rays without any electricity at all.
- Atomic number
- 69
- Atomic mass
- 168.93 u
- Electron configuration
- [Xe] 6s² 4f¹³
- Group · period · block
- f-block · 6 · f
- Oxidation states
- +3
- Electronegativity
- 1.25 (Pauling)
- Melting point
- 1,545 °C
- Boiling point
- 1,950 °C
- Density
- 9.32 g/cm³
- State at 20 °C
- Solid
- Appearance
- Silvery-grey metal
- Discovered
- Per Teodor Cleve, 1879
69 protons, and 69 electrons when neutral
IUPAC standard atomic weight
Shown in the strip below the table
What Thulium is used for
- Portable X-ray sources
- Surgical lasers
- High-temperature superconductors
- Euro banknote security markers
Atomic structure of Thulium
A neutral thulium atom has 69 protons and 69 electrons. Its most common isotope carries about 100 neutrons, which is where the mass number 169 comes from. Change the neutron count and you have a different isotope of thulium; change the proton count and you have a different element entirely.
Those electrons fill shells from the inside out: 2, 8, 18, 31, 8, 2. In orbital notation that is [Xe] 6s² 4f¹³. As a f-block element its valence count is variable, which is why it can form compounds in several oxidation states.
3D electron orbitals
See where Thulium's electrons actually are — a cloud of 26,000 points sampled from the probability density |ψ|², not a cartoon of a lobe.
Where Thulium sits in the periodic table
Discovery and naming
Thulium was discovered by Per Teodor Cleve in 1879. Its name comes from Thule, the ancient name for the far north, which is also why its symbol (Tm) does not match its English name.
Keep exploring
Atomic weight from IUPAC/CIAAW; physical properties from PubChem (US National Library of Medicine). Values that have never been measured are shown as “not measured” rather than estimated.
Related Calculators
Frequently Asked Questions
What is the atomic mass of Thulium?
The standard atomic weight of Thulium is 168.93 u, as published by IUPAC's Commission on Isotopic Abundances and Atomic Weights.
How many protons, neutrons and electrons does Thulium have?
Thulium has 69 protons, because the atomic number is the proton count and that is what makes an atom Thulium at all. A neutral Thulium atom has the same number of electrons, 69. The neutron count varies by isotope; the most common one has about 100, since 169 − 69 = 100.
What is the electron configuration of Thulium?
The electron configuration of Thulium is [Xe] 6s² 4f¹³. Its electrons are arranged 2, 8, 18, 31, 8, 2 across 6 shells.
Is Thulium a metal?
Yes. Thulium is classified as a lanthanide, on the metallic side of the table.
What group and period is Thulium in?
Thulium sits in the f-block strip below the main table, in period 6, among the lanthanides. Elements in that strip are not given a group number in the standard 18-column layout.
What is the charge of Thulium?
Thulium most commonly forms a 3+ ion. Like most transition metals it has several accessible oxidation states, so the charge depends on the compound.
What is the melting point of Thulium?
Thulium melts at 1,545 °C and boils at 1,950 °C. That puts it in the solids at everyday temperatures.
What is Thulium used for?
Portable X-ray sources, Surgical lasers, High-temperature superconductors, Euro banknote security markers. Thulium is the second rarest of the lanthanides and the least commercially used.
Who discovered Thulium, and where does its name come from?
Thulium (Tm) was discovered by Per Teodor Cleve in 1879. The name comes from Thule, the ancient name for the far north.