Thallium (Tl)
Stable isotopes of thallium available from ISOFLEX
| Isotope | Z(p) | N(n) | Atomic Mass | Natural Abundance | Enrichment Level | Chemical Form |
| Tl-203 | 81 | 122 | 202.972329 | 29.52% | >99.00% | Metal |
| Tl-203 | 81 | 122 | 202.972329 | 29.52% | >99.00% | Oxide |
| Tl-205 | 81 | 124 | 204.974412 | 70.48% | 99.90% | Metal |
| Tl-205 | 81 | 124 | 204.974412 | 70.48% | 99.90% | Oxide |
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Thallium was discovered in 1861 by Sir William Crookes. Its name derives from the Greek word thallos, meaning “green twig” or “green shoot.”
A bluish-white, lead-like solid, thallium has tetragonal crystals. It oxidizes in air at room temperature. It is soluble in nitric and sulfuric acid, insoluble in water (but readily forms soluble compounds when exposed to air or water), and slightly soluble in hydrochloric acid. It reacts with water containing oxygen to form thallous hydroxide, a relatively strong base, absorbing carbon dioxide and attacking glass. It burns in fluorine with incandescence. It reacts with other halogens to form halides. It also combines with several elements, forming binary compounds.
Thallium and its compounds have limited applications, including insecticides and rodenticides. Thallium-mercury alloys are used for switches and closures for use at sub-zero temperatures; another use is in low-melting glasses for electronic encapsulation. Thallium sulfide is used in photocells.
Thallium and its compounds (particularly its soluble salts) can cause serious or fatal poisoning from accidental ingestion or external application. Acute symptoms include nausea, vomiting, diarrhea, weakness, pain in extremities, convulsions and coma. Chronic effects include weakness, pain in extremities, and rapid loss of hair. Thallium and its compounds are listed under federal toxics regulations. It is listed by the United States Environmental Protection Agency as a priority pollutant metal in the environment.
Properties of Thallium
| Name | Thallium |
| Symbol | Tl |
| Atomic number | 81 |
| Atomic weight | 204.38 |
| Standard state | Solid at 298 ºK |
| CAS Registry ID | 7440-28-0 |
| Group in periodic table | 13 |
| Group name | None |
| Period in periodic table | 6 |
| Block in periodic table | p-block |
| Color | Silvery white |
| Classification | Metallic |
| Melting point | 303.5 °C |
| Boiling point | 1457 °C |
| Vaporization point | 1473 ºC |
| Thermal conductivity | 46.1 W/(m·K) at 298.2 °K |
| Electrical resistivity | 18.0 µΩ·cm at 0 °C |
| Electronegativity | 1.8 |
| Specific heat | 0.13 kJ/kg K |
| Heat of vaporization | 165 kJ·mol-1 |
| Heat of fusion | 4.2 kJ·mol-1 |
| Density of liquid | 11.22 g/cm3 |
| Density of solid | 11.85 g/cm3 |
| Electron configuration | [Xe]4f145d106s26p1 |
| Atomic radius | 1.70 Å |
| Oxidation states | +1 (thallous), +3 (thallic) |
