Thallium (Tl)
Post-transition metal · p-block · period 6 · group 13
Soft, heavy, bluish-grey metal that tarnishes to a dull grey.
Thallium (symbol Tl) is chemical element number 81, a post-transition metal in group 13, period 6 of the periodic table. Its standard atomic weight is 204.38 and its electron configuration is [Xe] 4f14 5d10 6s2 6p1. At room temperature it is a solid. Named after the bright green line in its spectrum ('thallos', a green shoot).
Properties
| Atomic number | 81 |
|---|---|
| Atomic mass | 204.38 |
| Category | Post-transition metal |
| Block | p-block |
| Group · Period | 13 · 6 |
| Electron configuration | [Xe] 4f14 5d10 6s2 6p1 |
| Electrons per shell | 2, 8, 18, 32, 18, 3 |
| State at 25 °C | solid |
| Melting point | 577 K |
| Boiling point | 1746 K |
| Density | 11.85 g/cm³ |
| Electronegativity (Pauling) | 1.62 |
| Radioactive | No |
| Origin | primordial |
| Discovered | 1861 · William Crookes |
Key facts about Thallium
- Named after the bright green line in its spectrum ('thallos', a green shoot).
- Highly toxic and once a favourite of poisoners — colourless, tasteless and slow-acting.
- Was widely used as rat and ant poison until it was banned in most countries.
- Thallium mixed with sulfur or selenium detects infrared light.
What Thallium is used for
Infrared detectors and optics, high-density glass, medical heart-imaging tracer, historically rodenticides.
Where it sits in the periodic table
Thallium is in group 13, period 6, of the p-block. Its electron configuration [Xe] 4f14 5d10 6s2 6p1 means its outermost shell holds 3 electrons, which is what places it here and explains much of its chemistry. See Thallium on the interactive table →
Thallium — frequently asked questions
What is Thallium used for?
Infrared detectors and optics, high-density glass, medical heart-imaging tracer, historically rodenticides.
Is Thallium radioactive?
No. Thallium has at least one stable isotope and is not radioactive.
What is the electron configuration of Thallium?
Thallium has the electron configuration [Xe] 4f14 5d10 6s2 6p1, which is 2, 8, 18, 32, 18, 3 electrons per shell.
What group and period is Thallium in?
Thallium is in group 13, period 6, in the p-block. Its outer shell holds 3 electrons.
Who discovered Thallium?
Thallium was discovered in 1861 by William Crookes.