Chemical element · Atomic number 81
Thallium
Thallium in the periodic table: atomic number 81, electron configuration, atomic mass, physical data, oxidation states, media credit and visible sources.
Post-transition metal
solid
204.383 u
Documented element sampleA documented sample of elemental thallium; the soft metal appears silvery-gray.
Image credit: James St. John
Auto-oriented, limited to 1600 × 1200 pixels and re-encoded as WebP; the subject was not altered.
Atomic classification
Shell occupancy
Thallium in the Bohr shell model
This shows the electron distribution of the neutral atom in a simplified shell model.
- K · n=1
- 2 electrons
- L · n=2
- 8 electrons
- M · n=3
- 18 electrons
- N · n=4
- 32 electrons
- O · n=5
- 18 electrons
- P · n=6
- 3 electrons
- Electron configuration
- [Xe]6s2 4f14 5d10 6p1
- Electrons per shell
- 2 · 8 · 18 · 32 · 18 · 3
- Group
- 13
- Period
- 6
- Block
- P
- Element category
- Post-transition metal
Physical and chemical properties
- Atomic mass
- 204.383 u
- Standard state
- solid
- Density
- 11.8 g/cm³
- Melting point
- 577 K
- Boiling point
- 1,746 K
- Electronegativity
- 1.62 (Pauling)
- First ionisation energy
- 6.108 eV
- Oxidation states
- +3, +1
- Discovery
- 1861
Safety and periodic classification
Safety
Safe handling cannot be inferred from Thallium's position in the periodic table alone. Laboratory, classroom and disposal decisions must follow the documentation for the exact material and its safety data sheet.
Position and comparison
Thallium is in period 6, group 13 and the P block. Its direct neighbours by atomic number are Mercury and Lead. The recorded Pauling electronegativity is 1.62. Periodic trends are compared only through the separately sourced neighbouring values.
Sources and scope
PubChem attributes element data to sources including IUPAC, NIST and IAEA. Quanta stores the referenced snapshot locally and leaves unknown values unavailable.
Where Thallium sits among the 118 elements
The table compares the reported values for Thallium with every other element that carries the same value in the snapshot. A rank is not a judgement, it is orientation: it says how unusual this value is across the periodic table.
| Value | Thallium | Position in the data set |
|---|---|---|
| Melting point | 577 K | Rank 74 of 103 elements with a reported value. Highest: Carbon. |
| Boiling point | 1,746 K | Rank 62 of 93 elements with a reported value. Highest: Rhenium. |
| Density | 11.8 g/cm³ | Rank 20 of 96 elements with a reported value. Highest: Osmium. |
| Electronegativity (Pauling) | 1.62 | Rank 48 of 95 elements with a reported value. Highest: Fluorine. |
| Ionisation energy | 6.108 eV | Rank 73 of 102 elements with a reported value. Highest: Helium. |
All values and ranks come from the PubChem Periodic Table snapshot, retrieved on 2026-07-15. Elements without a reported value are excluded, so the population differs per row.
The neighbours of Thallium in the periodic table
- Place in the table
- Group 13, period 6, P block. Classified as post-transition metal, solid at standard conditions.
- Direct neighbours
- Atomic number 81. Preceded by Mercury, followed by Lead.
- Same group
- Same number of outer electrons, and therefore similar reactivity to Boron, Aluminum, Gallium, Indium, Nihonium.
- Discovery
- Recorded in the snapshot as: 1861. Who, where and under what circumstances is not part of that record, so this page carries the sourced year and no discovery narrative.
- Oxidation states
- +3, +1. They follow from the electron configuration [Xe]6s2 4f14 5d10 6p1 and indicate which compounds to expect.
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