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Thallium

Element 81 · Post-transition metal

Specimen

Atomic number
81
Atomic mass
204.38
State at room temperature
Solid
Melting point
303.9 °C
Boiling point
1,473 °C
Density
11.8 g/cm³
Discovered
1861

Atomic mass from IUPAC.1 Other physical data from PubChem.2

Can I eat it?

In food and water
Poison. Thallium salts are very toxic. Poisoning by mouth harms the nerves and the gut, causes hair loss, and can kill. Thallium takes the place of potassium in the body. 34

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The atom

Tl
ShellElectrons
1 (K) 2
2 (L) 8
3 (M) 18
4 (N) 32
5 (O) 18
6 (P) outermost 3
Total81

Configuration [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹

4f ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓
5d ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓
6s ↑ ↓
6p ↑
A shell diagram: each ring is an electron shell and each dot is one electron. The nucleus holds 81 protons. The outermost shell is shown in red. Electrons do not travel on rings: they fill orbitals, regions where an electron is likely to be found. The boxes are those orbitals for the electrons outside the Xe core, with one arrow per electron.

Related elements

Group 13
B Boron Al Aluminium Ga Gallium In Indium Nh Nihonium

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Sources

  1. 1

    IUPAC Commission on Isotopic Abundances and Atomic Weights (CIAAW), 2024. Abridged Standard Atomic Weights 2024 .

    The atomic mass shown for the 84 elements that have a standard atomic weight.

  2. 2

    National Center for Biotechnology Information (PubChem), 2026. Periodic Table of Elements .

    Electron configuration, melting and boiling points, density and discovery year for all 118 elements.

  3. 3

    Agency for Toxic Substances and Disease Registry (ATSDR), 2024. Toxicological Profile for Thallium (Draft for Public Comment), Chapter 1: Relevance to Public Health , Chapter 1, dermal effects.

    • Alopecia (hair loss) is a classic symptom of acute thallium poisoning
    • Body weight decreases, dermal effects, nervous system effects, gastrointestinal effects, and death were the most sensitive targets following oral exposure to thallium
  4. 4

    Royal Society of Chemistry, Periodic Table, 2026. Thallium - Element information, properties and uses .

    Reference page; its Biological role and Natural abundance sections are quoted.

    • Thallium has no known biological role. It is very toxic ... It can displace potassium around the body
  5. 5

    National Institute of Standards and Technology (NIST), 2026. Atomic Weights and Isotopic Compositions for All Elements .

    Isotopic composition of each element; the most abundant isotope sets the neutron count in the nucleus view.