Tl
Thallium
Element 81 · 204.38 u
Thallium is a highly toxic post-transition metal once used as rat poison.
Physical Properties
| Atomic Mass | 204.38 u |
| Density | 11.85 g/cm³ |
| Melting Point | 577.0 K |
| Boiling Point | 1746.0 K |
| Appearance | Silvery-white metallic |
| State at Room Temp | Solid |
Chemical Properties
| Electronegativity | 1.62 (Pauling) |
| 1st Ionization Energy | 589.4 kJ/mol |
| Electron Affinity | 19.2 kJ/mol |
| Oxidation States | +1, +3 |
Atomic Properties
| Electron Configuration | [Xe] 4f14 5d10 6s2 6p1 |
| Atomic Radius | 170.0 pm |
| Covalent Radius | 145.0 pm |
| Van der Waals Radius | 196.0 pm |
Discovery
| Discovered By | William Crookes |
| Discovery Year | 1861 |
| Location | London, England |
| Named After | Greek 'thallos' meaning green shoot, from its bright green spectral line |
About Thallium
Thallium is a soft, silvery-white metal that is extremely toxic. It was historically used as a murder weapon due to its tastelessness and was banned as rat poison in many countries.
Uses & Applications
Semiconductor devices, infrared optics, medical imaging, and specialized glass.
Fun Fact
Thallium was the 'poisoner's poison' -- tasteless, odorless, and its symptoms mimicked common illnesses.
Isotopes
| Mass Number | Abundance | Half-Life | Stable |
| 203 | 0.2952% | - | Yes |
| 205 | 0.7048% | - | Yes |
Electron Configuration
[Xe] 4f14 5d10 6s2 6p1