Thallium
post transition metalProperties
| Property | Value |
|---|---|
| Atomic Mass | 204.38 amu |
| Category | post transition metal |
| Group | 13 |
| Period | 6 |
| Electron Configuration | [Xe] 4f14 5d10 6s2 6p1 |
| Electronegativity | 1.62 (Pauling) |
| Oxidation States | 3, 1 |
| Melting Point | 577 K (303.9 °C) |
| Boiling Point | 1746 K (1472.8 °C) |
| Density | 11.85 g/cm³ |
| Discovered By | William Crookes (1861) |
About Thallium
Crookes spotted thallium in 1861 as a single bright green line in a flame spectrum — λ ≈ 535 nm — coming off residues from a sulfuric acid plant, and named it after the Greek for green shoot. The 6s² inert pair sits low enough that Tl(I) dominates aqueous chemistry; Tl⁺ has nearly the same ionic radius as K⁺ (≈ 1.50 vs 1.52 Å) and slips into K⁺ channels and pumps in cells, which is why the colorless, tasteless Tl₂SO₄ became infamous in mid-century rodenticides and poisoning cases. That same K⁺-mimicry is what makes ²⁰¹Tl-chloride a useful myocardial perfusion tracer: viable heart muscle takes it up exactly the way it pulls in potassium, so SPECT images of Tl distribution map blood flow and viability. NaI(Tl) crystals — sodium iodide doped with about 0.1% thallium — remain the standard scintillator for gamma spectroscopy in everything from well-logging tools to airport monitors.
Fun Fact
Thallium was once called 'the poisoner's poison' because its salts are colorless, odorless, and tasteless — Agatha Christie famously used thallium as the murder weapon in her novel 'The Pale Horse,' and the book later helped solve a real poisoning case.
Common Uses
- ²⁰¹Tl-chloride myocardial perfusion SPECT for ischemia and viability imaging
- NaI(Tl) and CsI(Tl) scintillator crystals in gamma spectrometers and PET shields
- TlBr and TlBr-TlI (KRS-5) optics for 0.6–40 µm IR spectroscopy and CO₂-laser windows
- Tl-Ba-Ca-Cu-O cuprate superconductors with Tc above 125 K for research magnets
- High-density (3.5 g/mL) Tl-formate/malonate solutions for mineral float separation
- Low-melting Hg-Tl eutectic (−60 °C) thermometers for cryogenic work