ChemSphere
69Tm168.934

Thulium

Lanthanide · Period 6 · f-block

The rarest stable lanthanide, named for the far north

Atomic mass
168.934 u
State at 25 °C
Solid
Electronegativity
1.25
Melting point
1,544.9 °C
Boiling point
1,949.9 °C
Configuration
[Xe] 4f13 6s2
Building atom…
69 protons 100 neutrons 69 electrons

Nucleus drawn at reduced count for clarity. Drag to rotate.

Thulium, explained

Video lesson in production

Bhavya’s explainer for this element will play here.

Overview

Thulium is the least abundant of the naturally occurring lanthanides, at about half a part per million in the crust. Its configuration is [Xe]4f13 6s2, so the Tm3+ ion has a 4f shell just one electron short of being full. Like other lanthanides it is a soft, silvery metal that tarnishes slowly in air, and its salts are pale green. A divalent state, Tm2+, also exists in a few compounds, though less readily than for its neighbour ytterbium.

Thulium's applications come from its nucleus and its electrons. Bombarding thulium-169, its only stable isotope, with neutrons produces thulium-170, a gamma emitter with a 128-day half-life used as the source in portable X-ray devices. Thulium-doped lasers emit around 2 micrometres, a wavelength strongly absorbed by water, which surgeons use to vaporise prostate tissue and break up stones. Thulium-doped fibres are also used to amplify optical signals in the S-band of telecommunications.

Discovery

Swedish chemist Per Teodor Cleve discovered thulium in 1879 while studying impurities in erbium oxide, separating a brown fraction he named holmia and a green fraction he named thulia. He named thulium after Thule, the ancient name for a mythical land in the far north. Pure thulium compounds were extremely hard to obtain: in 1911 American chemist Charles James performed about 15,000 recrystallisations of thulium bromate to get a pure sample.

Where it’s found
About 0.5 parts per million of Earth's crust, extracted mainly from monazite and ion-adsorption clays.
What it’s used for
  • Portable X-ray sources
  • Surgical thulium lasers
  • Fibre-optic amplifiers
  • Ceramic magnetic materials
  • Research
In living things
No known biological role; it has low toxicity.
Safety
Bulk metal is relatively safe; powders can ignite, and thulium-170 sources are radioactive and need shielding.

How it compares

Where Thulium sits among all 118 elements on the trends that shape the table.

  • Electronegativity1.25Pauling

    #74 of 100 elements with data

  • Ionization energy597kJ/mol

    #69 of 104 elements with data

  • Atomic radius175pm

    #25 of 95 elements with data

  • Electron affinity99kJ/mol

    #30 of 109 elements with data

  • Density9.32g/cm³

    #43 of 114 elements with data

  • Melting point1,820K

    #25 of 110 elements with data

Density
9.32 g/cm³
Atomic radius
175 pm
Covalent radius
164 pm
Molar heat
27.03 J/(mol·K)
Appearance
silvery gray
Discovered by
Per Teodor Cleve

Inside the atom

69 electrons arranged in 6 shells: 2 · 8 · 18 · 31 · 8 · 2.

Orbital diagram

1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f13 · shorthand [Xe] 4f13 6s2

  1. 1s2
  2. 2s2
  3. 2p6
  4. 3s2
  5. 3p6
  6. 4s2
  7. 3d10
  8. 4p6
  9. 5s2
  10. 4d10
  11. 5p6
  12. 6s2
  13. 4f13

Each box is an orbital holding up to two electrons of opposite spin. Highlighted subshells hold the valence electrons that decide how Thulium bonds.

Ionization energies

Energy to remove each electron in turn, on a log scale. Each electron is harder to remove than the last, because the remaining ones feel more of the nucleus.

Signature reactions

The chemistry Thulium is known for.

  1. Redox · Combustion

    Burning in air

    Thulium burns to form a pale green oxide. The oxide is the form in which thulium is purified and traded.

    4Tm(s)+3O2(g)2Tm2O3(s)

    Conditions: heating in air

  2. Redox · Single displacement

    Reaction with water

    Thulium reacts slowly with water to produce hydrogen and thulium hydroxide. The reaction is sluggish, consistent with thulium's position near the end of the lanthanide series.

    2Tm(s)+6H2O(l)2Tm(OH)3(s)+3H2(g)

    Conditions: slow in cold water, faster when hot

  3. Nuclear · Neutron capture

    Neutron activation

    Stable thulium-169 captures a neutron to become radioactive thulium-170. This isotope emits low-energy gamma rays, making it a compact source for portable radiography.

    169Tm+10n170Tm

    Conditions: neutron flux in a nuclear reactor

Where you’ll meet it

Research

No research on Thulium yet

When Bhavya publishes a project that studies Thulium, it will be linked here.

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