About

Thallium is a soft, grayish post-transition metal that resembles tin but discolors on exposure to air. It is extremely toxic to humans and was historically used as a potent rat poison and insecticide, but this is now banned in most countries. It has niche uses in specialized electronics, low-temperature thermometers, and in lenses for infrared optics.

Atomic Properties

Atomic Number81

Atomic Weight204.383 u

Neutrons (common)124

Electron Configuration[Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹

Full Configuration1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 6s² 6p¹

Electrons per Shell2, 8, 18, 32, 18, 3

Electronegativity1.62

Electron Affinity19.2 kJ/mol

Oxidation States+3, +1

Covalent Radius148 pm

Van der Waals Radius196 pm

Ionic Radius150 pm

Periodic Table Position

Group13

Period6

BlockP

Chemical SeriesPost-transition metals

Physical Properties

State at STPsolid

Appearancesilvery-white metal that tarnishes to a bluish-gray on exposure to air

Crystal Structurehcp

Magnetic Orderingdiamagnetic

Speed of Sound818 m/s

Poisson Ratio0.45

Thermal Properties

Melting Point577 K

Boiling Point1,746 K

Triple Point577 K

Thermal Expansion0 1/K

Thermal Conductivity46.1 W/(m·K)

Specific Heat0.129 J/(g·K)

Molar Heat26.37 J/(mol·K)

Heat of Fusion4.14 kJ/mol

Heat of Vaporization165 kJ/mol

Abundance

In Earth's Crust0.85 ppm

In Ocean0 ppm

In Solar System0 ppm

In Universe0.001 ppm

Discovery

Discovered byWilliam Crookes, Claude-Auguste Lamy

Discovery Year1,861

Occurrenceprimordial

CAS Number7440-28-0

Electron Configuration

Energy

↿⇂

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Isotopes of Thallium

Symbol ▲	Mass (u) ▲	Abundance ▲	Half-life ▲	Decay Mode ▲
²⁰¹Tl	200.970820	—	72.912 h	Electron Capture
²⁰²Tl	201.972100	—	12.31 d	Electron Capture, Beta Minus Decay
²⁰³Tl	202.972340	29.515%	Stable	Stable
²⁰⁴Tl	203.973872	—	3.783 y	Beta Minus Decay, Electron Capture
²⁰⁵Tl	204.974420	70.485%	Stable	Stable

Interactive Visualization

Explore the 3D Bohr model of Thallium in our interactive viewer.

View 3D Model of Thallium