Thallium

01 / A closer look

Meet Thallium

Thallium has atomic number 81: every atom of this element contains 81 protons. It belongs to period 6, group 13 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

577 K

Boiling point

1746 K

Density

11.8 g/cm³

Atomic weight

204.38 ± 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 4f14 5d10 6p1

Oxidation states

+3, +1

Electronegativity · Pauling

1.62

First ionisation energy

6.108 eV

Atomic radius

196 pm

04 / Where it is found

Where it is found

Recovered as a by-product of processing other metal ores.

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