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
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) 

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