Thallium (Tl)
Element 81 · Post-transition metal · atomic mass 204.38 u · group 13, period 6
Electron shells: 2, 8, 18, 32, 18, 3
Thallium is odourless, tasteless and extremely toxic, which earned it the nickname "the poisoner's poison". Legitimate uses are now confined to specialist optics and electronics.
Agatha Christie described thallium poisoning so accurately that a nurse later used one of her novels to identify a real case.
- Atomic number
- 81
- Atomic mass
- 204.38 u
- Electron configuration
- [Xe] 6s² 4f¹⁴ 5d¹⁰ 6p¹
- Group · period · block
- 13 · 6 · p
- Oxidation states
- +1, +3
- Electronegativity
- 1.62 (Pauling)
- Melting point
- 304 °C
- Boiling point
- 1,473 °C
- Density
- 11.8 g/cm³
- State at 20 °C
- Solid
- Appearance
- Soft, grey metal
- Discovered
- William Crookes, 1861
81 protons, and 81 electrons when neutral
IUPAC interval [204.382, 204.385]
Boron group
What Thallium is used for
- Infrared optical glass
- High-temperature superconductors
- Cardiac imaging isotopes
- Semiconductor detectors
Atomic structure of Thallium
A neutral thallium atom has 81 protons and 81 electrons. Its most common isotope carries about 123 neutrons, which is where the mass number 204 comes from. Change the neutron count and you have a different isotope of thallium; change the proton count and you have a different element entirely.
Those electrons fill shells from the inside out: 2, 8, 18, 32, 18, 3. In orbital notation that is [Xe] 6s² 4f¹⁴ 5d¹⁰ 6p¹, leaving 3 valence electrons in the outer shell — the ones that do the chemistry.
3D electron orbitals
See where Thallium's electrons actually are — a cloud of 26,000 points sampled from the probability density |ψ|², not a cartoon of a lobe.
Where Thallium sits in the periodic table
Metals to the right of the d-block. Softer, lower-melting and more brittle than the transition metals, with more covalent character in their bonding.
Discovery and naming
Thallium was discovered by William Crookes in 1861. Its name comes from Greek thallos, "green shoot", which is also why its symbol (Tl) does not match its English name.
Keep exploring
Atomic weight from IUPAC/CIAAW; physical properties from PubChem (US National Library of Medicine). Values that have never been measured are shown as “not measured” rather than estimated.
Related Calculators
Frequently Asked Questions
What is the atomic mass of Thallium?
The standard atomic weight of Thallium is 204.38 u. Because the natural isotope mix varies between sources, IUPAC publishes it as the interval [204.382, 204.385], and 204.38 is the conventional single value to use in calculations.
How many protons, neutrons and electrons does Thallium have?
Thallium has 81 protons, because the atomic number is the proton count and that is what makes an atom Thallium at all. A neutral Thallium atom has the same number of electrons, 81. The neutron count varies by isotope; the most common one has about 123, since 204 − 81 = 123.
What is the electron configuration of Thallium?
The electron configuration of Thallium is [Xe] 6s² 4f¹⁴ 5d¹⁰ 6p¹. Its electrons are arranged 2, 8, 18, 32, 18, 3 across 6 shells.
Is Thallium a metal?
Yes. Thallium is classified as a post-transition metal, on the metallic side of the table.
What group and period is Thallium in?
Thallium is in group 13 (the boron group), period 6, in the p-block. The group is its column and the period is its row.
What is the charge of Thallium?
Thallium most commonly forms a 1+ or 3+ ion. It has 3 outer electrons available to lose.
What is the melting point of Thallium?
Thallium melts at 304 °C and boils at 1,473 °C. That puts it in the solids at everyday temperatures.
What is Thallium used for?
Infrared optical glass, High-temperature superconductors, Cardiac imaging isotopes, Semiconductor detectors. Thallium is odourless, tasteless and extremely toxic, which earned it the nickname "the poisoner's poison".
Who discovered Thallium, and where does its name come from?
Thallium (Tl) was discovered by William Crookes in 1861. The name comes from Greek thallos, "green shoot".