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Thulium

Element 69 · Lanthanide

Specimen

Atomic number
69
Atomic mass
168.93
State at room temperature
Solid
Melting point
1,545 °C
Boiling point
1,950 °C
Density
9.32 g/cm³
Discovered
1879

Atomic mass from IUPAC.1 Other physical data from PubChem.2

Can I eat it?

In food and water
Not needed, little harm known. Thulium has no known function in the body. The RSC calls it non-toxic. In rats, half died only at 6.25 g of thulium chloride per kg of body weight. There are few studies of it in food. 34

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The atom

Tm
ShellElectrons
1 (K) 2
2 (L) 8
3 (M) 18
4 (N) 31
5 (O) 8
6 (P) outermost 2
Total69

Configuration [Xe] 4f¹³ 6s²

4f ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑
6s ↑ ↓
A shell diagram: each ring is an electron shell and each dot is one electron. The nucleus holds 69 protons. The outermost shell is shown in red. Electrons do not travel on rings: they fill orbitals, regions where an electron is likely to be found. The boxes are those orbitals for the electrons outside the Xe core, with one arrow per electron.

Related elements

Lanthanides
La Lanthanum Ce Cerium Pr Praseodymium Nd Neodymium Pm Promethium Sm Samarium Eu Europium Gd Gadolinium Tb Terbium Dy Dysprosium Ho Holmium Er Erbium Yb Ytterbium Lu Lutetium

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Sources

  1. 1

    IUPAC Commission on Isotopic Abundances and Atomic Weights (CIAAW), 2024. Abridged Standard Atomic Weights 2024 .

    The atomic mass shown for the 84 elements that have a standard atomic weight.

  2. 2

    National Center for Biotechnology Information (PubChem), 2026. Periodic Table of Elements .

    Electron configuration, melting and boiling points, density and discovery year for all 118 elements.

  3. 3

    Royal Society of Chemistry, 2026. Thulium - Element information, properties and uses .

    • Thulium has no known biological role. It is non-toxic.
  4. 4

    Toxicology Excellence for Risk Assessment (TERA), for the US Bureau of Land Management, 1999. Development of Reference Doses and Reference Concentrations for Lanthanides , Section 2.4 (oral toxicity data in experimental animals).

    • 6250 mg/kg for thulium chloride
  5. 5

    National Institute of Standards and Technology (NIST), 2026. Atomic Weights and Isotopic Compositions for All Elements .

    Isotopic composition of each element; the most abundant isotope sets the neutron count in the nucleus view.