Thallium (Tl)

Pure, unreacted thallium appears silvery-white and exhibits a metallic lustre. Upon reacting with air, it begins to turn bluish-grey and looks like lead. It is very malleable, and can be cut with a knife. There are two stable isotopes, and four radioisotopes, Tl-204 being the most stable with a half-life of 3.78 years. Thallium sulphate was used as a rodenticide. Thallium sulphine's conductivity changes with exposure to infrared light, this gives it a use in infrared detectors. Discovered by Sir William Crookes via spectroscopy. Its name comes from the Greek word thallos, which means green twig. Thallium and its compounds are toxic and can cause cancer.
Atomic Number81
Atomic Weight204.38
Mass Number205
Protons81 p+
Neutrons124 n0
Electrons81 e-
Thallium pieces in ampoule.jpg Thallium-croprotated.jpg Animated Bohr Model Enhanced Bohr Model Bohr Model Orbital Diagram


Atomic Radius
190 pm
Atomic Volume
Covalent Radius
144 pm
Metallic Radius
144 pm
Ionic Radius
150 pm
Crystal Radius
164 pm
Van der Waals radius
196 pm
11.8 g/cm³
Boiling Point
1,730 K
Melting Point
576.6 K
Electrons per shell2, 8, 18, 32, 18, 3
0.9173102715055 eV/particle
Proton Affinity
Electron Affinity
Ionization Potential
6.108287 eV/particle
Heat of Vaporization
162.4 kJ/mol
Heat of Fusion
4.31 kJ/mol
Heat of Formation
182.2 kJ/mol
Molar Heat Capacity
26.32 J/(mol K)
Specific Heat Capacity
0.129 J/(g⋅K)
Thermal Conductivity
46.1 W/(m K)
Gas Basicity
Dipole Polarizability
50 a₀
C6 Dispersion Coefficient
Oxidation States1, 3
Crystal StructureSimple Hexagonal (HEX)
Lattice Constant
3.46 Å
Bulk Modulus
Electrical Resistivity
Electron Configuration[Xe] 4f14 5d10 6s2 6p1
Magnetic Ordering
Magnetic Susceptibility
Poisson Ratio
Shear Modulus
Young's Modulus
Alternate Names
Adiabatic Index
Electric Conductivity
Critical Pressure
Critical Temperature
Curie Point
Magnetic Susceptibility
Neel Point
Neutron Cross Section
Neutron Mass Absorption
Gas Phase
Quantum Numbers
Refractive Index
Space Group
Speed of Sound
Superconducting Point
Thermal Expansion
Valence Electrons
CategoryPost-transition metals, Poor metals
Glawe Number81
Mendeleev Number85
Pettifor Number78
Geochemical Class
Goldschmidt Classchalcophile
Decay Mode
Abundance in Earth's crust
0.85 mg/kg
Abundance in Oceans
0.000019 mg/L
Abundance in Human Body
Abundance in Meteor
Abundance in Sun
Abundance in Universe5×10-8%

Isotopes of Thallium

Stable Isotopes
203Tl 205Tl
Unstable Isotopes
176Tl 177Tl 178Tl 179Tl 180Tl 181Tl 182Tl 183Tl 184Tl 185Tl 186Tl 187Tl 188Tl 189Tl 190Tl 191Tl 192Tl 193Tl 194Tl 195Tl 196Tl 197Tl 198Tl 199Tl 200Tl 201Tl 202Tl 204Tl 206Tl 207Tl 208Tl 209Tl 210Tl 211Tl 212Tl


Thallium was discovered spectroscopically by Sir William Crookes in 1861, in London. In 1862, Claude-Auguste Lamy used a spectrometer to determine the composition of a selenium-containing substance which was deposited during the production of sulfuric acid from pyrite. He noticed the new green line in the spectra and concluded that a new element was present. From Greek thallos, meanin a green shoot or twig

DiscoverersSir William Crookes
Discovery LocationEngland
Discovery Year1861
Name OriginGreek: thallos (green twig), for a bright green line in its spectrum.
Thallium and its compounds are highly toxic
Agatha Christie made use of thallium's toxicity in her novel The Pale Horse


Thallium selenide has been used in a bolometer for infrared detection. Thallium is also used in gamma radiation detection equipment. Thallium oxide has been used to produce glasses with a high index of refraction, and is used in the manufacture of photo cells. Some of the electrodes in dissolved oxygen analyzers contain thallium. Its compounds are used in rat and ant poisons. Also for detecting infrared radiation.


Found in iron pyrites. Also in crookesite, hutchinsonite and lorandite. Most is recovered from the byproducts of lead and zinc refining.