About

Thulium is the least abundant of the stable rare-earth elements and the second-least abundant of all lanthanides. When bombarded with neutrons in a nuclear reactor, it produces an isotope that emits X-rays, making it useful in portable, non-electric X-ray devices for medical and industrial purposes. It has a bright, silvery luster but is not widely used due to its high cost.

Atomic Properties

Atomic Number69

Atomic Weight168.934 u

Neutrons (common)100

Electron Configuration[Xe] 4f¹³ 6s²

Full Configuration1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹³ 5s² 5p⁶ 6s²

Electrons per Shell2, 8, 18, 31, 8, 2

Electronegativity1.25

Electron Affinity99 kJ/mol

Oxidation States+3, +2

Covalent Radius156 pm

Van der Waals Radius222 pm

Ionic Radius88 pm

Periodic Table Position

Group3

Period6

BlockF

Chemical SeriesLanthanoids

Physical Properties

State at STPsolid

Appearancebright, silvery-gray

Crystal Structurehcp

Magnetic Orderingparamagnetic

Speed of Sound2,655 m/s

Poisson Ratio0.213

Thermal Properties

Melting Point1,818 K

Boiling Point2,223 K

Triple Point1,818 K

Thermal Expansion0 1/K

Thermal Conductivity16.9 W/(m·K)

Specific Heat0.16 J/(g·K)

Molar Heat27.03 J/(mol·K)

Heat of Fusion16.84 kJ/mol

Heat of Vaporization232 kJ/mol

Abundance

In Earth's Crust0.52 ppm

In Ocean0 ppm

In Solar System0 ppm

In Universe0 ppm

Discovery

Discovered byPer Teodor Cleve

Discovery Year1,879

Occurrenceprimordial

CAS Number7440-30-4

Electron Configuration

Energy

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Isotopes of Thulium

Symbol ▲	Mass (u) ▲	Abundance ▲	Half-life ▲	Decay Mode ▲
¹⁶⁹Tm	168.934210	100%	Stable	Stable

Interactive Visualization

Explore the 3D Bohr model of Thulium in our interactive viewer.

View 3D Model of Thulium