Thallium
Element 81 · Post-Transition Metal
Thallium is a soft metal you could cut with a knife, gray with a bluish tinge, sitting just past mercury in period 6. Its chemistry is dominated by the +1 state, whose ion mimics potassium — and that mimicry is what makes thallium so dangerous, letting it slip into biological systems.
Historically it was used in rat poisons and insecticides, now banned in most countries. Remaining legitimate uses are narrow: infrared optics, specialized electronics, and low-temperature thermometers, all under tight controls.
Key facts
| Atomic number | 81 |
|---|---|
| Atomic weight | 204.38 |
| Category | Post-Transition Metal |
| Group / Period | 13 / 6 |
| Block | p |
| Electron configuration | 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p1 |
| Common oxidation states | +1, +3 |
| State at STP | Solid |
| Electronegativity | 1.62 |
| Density | 11.85 g/cm³ |
| Melting point | 304 °C |
| Boiling point | 1473 °C |
Occurrence
Thallium occurs only in traces, scattered through sulfide ores such as pyrite and sphalerite, and is recovered as a byproduct of smelting other metals.
Applications
Infrared-transmitting optics
Specialized electronic components
Low-temperature thermometers (historic)
Rat poison (historic, banned)
Common compounds
- thallium(I) chloride
- thallium(I) oxide
Thallium was discovered through a bright green line in its spectrum — its name comes from the Greek thallos, meaning ‘green shoot’.
Data source: IUPAC standard atomic weights 2021