Thulium (Tm)

Thulium is the least abundant of the stable lanthanides, though still more common in the crust than gold or platinum. Thulium-170, made by irradiating the stable isotope in a reactor, emits soft X-rays and needs no power supply, which makes it useful for radiography where there is no electricity — inspecting pipeline welds in the field, for instance. It is named after Thule, the classical name for the far north.

Properties

Atomic number

69

Symbol

Tm

Standard atomic weight

168.93 u

Category

Lanthanide

Period

6

Block

f-block

Electron configuration

[Xe] 6s² 4f¹³

Electrons per shell

2, 8, 18, 31, 8, 2

Melting point

1,818 K — 1544.8 °C — 2812.7 °F

Boiling point

2,223 K — 1949.8 °C — 3541.7 °F

Density

9.32 g/cm³

Electronegativity

1.25 (Pauling)

Oxidation states

+3

Discovered

1879 by Per Teodor Cleve

Electron configuration

Full

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

Noble gas shorthand

[Xe] 6s² 4f¹³

Shells

K: 2 · L: 8 · M: 18 · N: 31 · O: 8 · P: 2

What it is used for

• Thulium-170 as a portable X-ray source for field radiography
• Thulium fibre lasers for surgery and materials processing
• Blue phosphors in euro banknote security features
• Dopant in high-temperature superconductors

Neighbours in the table