Thulium

01 / A closer look

Meet Thulium

Thulium has atomic number 69: every atom of this element contains 69 protons. It belongs to period 6 of the periodic table.

In a neutral atom, the number of electrons equals the number of protons. Isotopes of this element share the same atomic number but have different neutron counts.

02 / Physical properties

Melting point

1818 K

Boiling point

2223 K

Density

9.32 g/cm³

Atomic weight

168.93 ± 0.01

Melting points, boiling points and densities follow RSC fact boxes. Density is referenced to room temperature; consult the source for pressure and allotrope details. These are reference values, not a claim that every value was experimentally measured. Atomic weights follow CIAAW.

Physical-property source: Royal Society of Chemistry ↗ · Reviewed 24 September 2026

Standard atomic weights follow CIAAW 2024 (abridged). Brackets show an isotope mass number from IUPAC 2022, not a standard atomic weight.

03 / Atomic structure

Atomic structure

Electron configuration

[Xe]6s2 4f13

Oxidation states

+3

Electronegativity · Pauling

1.25

First ionisation energy

6.184 eV

Atomic radius

227 pm

04 / Where it is found

Where it is found

Small quantities in rare-earth minerals.

Production and export figures depend on the commodity and reporting year. No comparable country quantities have been included for this element in this edition.