Thulium (Tm)

Silvery gray

Thulium is the least abundant lanthanide, used in portable X-ray machines and golf club faces, and with thirteen 4f electrons, Tm³⁺ emits at 2 μm in the eye-safe wavelength range. Thulium-doped lasers are used in surgical procedures and portable X-ray sources where Tm-170 emits characteristic X-rays.

Key metrics

Density9.32g/cm³

Melting point1,818K

Boiling point2,223K

Electronegativity1.25Pauling

Discovery

1879discovered

Discovered by: Per Teodor Cleve
Reactivity profile: Stable in dry air
Reactivity: Medium
Bonding: Metallic

Electron configuration

\textcolor{#6b7280}{[Xe]}\; \textcolor{#a855f7}{4_{f}^{13}}\; \textcolor{#ef4444}{6_{s}^{2}}

[Xe] 4f13 6s2

Wave function

\Psi_{\text{Tm}} \approx \frac{1}{\sqrt{69!}} \det \left| \chi_1(\mathbf{r}_1) \dots \chi_{69}(\mathbf{r}_{69}) \right|

Probability density $|\Psi|^2$ across the Tm valence shell.

Crystal structure

Hexagonal Close-Packed

Lattice constantsa 1.00 · b 1.00 · c 1.63

Cell anglesα 90° · β 90° · γ 120°

Coordination number12

Atoms per cell2

Atomic packing factor0.74

Physical properties

Atomic radius: 172 pm
Ionization energy: 597 kJ/mol
Electron affinity: 50 kJ/mol
Crystal structure: Hexagonal close-packed (hcp)

Oxidation states

Acid/base

Basic

Strongly acidicStrongly basic

State at 25 °CSolid

1818 K2223 K298 K

Emission spectrum

3 transitions

380450500550600650700780

VioletBlueGreenYellowRed

410.6nmTm I (violet)Violet60%

456.0nmTm I (blue)Blue40%

530.7nmTm I (green)Green100%

Dominant line 530.7 nmRange 411–531 nmTransitions 3