81
Tl
Thallium
205
Thallium (Tl)
Post-Transition MetalGroup 13Period 6p-block-block
Thallium is a chemical element with symbol Tl and atomic number 81. This soft gray post-transition metal is not found free in nature. When isolated, it resembles tin, but discolors when exposed to air.
Latin name: Thallium
Atomic Number
81
Atomic Mass
205 g/mol
Phase (STP)
Solid
Electron Config.
[Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹
General
SymbolTl
Atomic Number81
Atomic Mass205 g/mol
CategoryPost-Transition Metal
Group13
Period6
Blockp-block
Electron Configuration[Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹
Valence Electrons13
Coloursilvery white metallic
Appearancesilvery white
CAS NumberCAS7440-28-0
Physical Properties
Phase (STP)Solid
Density11.85 g/cm³
Melting Point303.85°C · 578.93°F · 577.00 K
Boiling Point1472.85°C · 2683.13°F · 1746.00 K
Atomic Properties
Atomic Radius196 pm
Electronegativity1.62 Pauling
Ionization Energy589 kJ/mol
Oxidation States+1, +3
Ion ChargeTl+/Tl3+
Thermodynamic
Molar Heat26.32 J/(mol·K)
Crystal Structure
Crystal Structurehexagonal close-packed
Mechanical Properties
Mohs Hardness1.5
Nuclear & Hazards
Half-lifestable (natural isotopes)
Abundance
In Universe0.000003%
In Sun0.0000025%
In Earth's Crust0.00005999999999999999%
Discovery & History
Discovery Year1861
Discovered ByWilliam Crookes
Country of DiscoveryEngland
Isotopes of Thallium
| Isotope | Abundance | Half-life | Decay |
|---|---|---|---|
| Tl-203 | 29.524% | Stable | — |
| Tl-204 | — | 3.783 years | β⁻, EC |
| Tl-205 | 70.476% | Stable | — |