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Thorium

Element 90 · Actinide

Specimen

Atomic number
90
Atomic mass
232.04
State at room temperature
Solid
Melting point
1,750 °C
Boiling point
4,788 °C
Density
11.72 g/cm³
Discovered
1828

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

Can I eat it?

In food and water
Radioactive. All thorium is radioactive. Food and water contain very low levels, and the body takes up little of it. Animals that breathed thorium got lung cancer. 34

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The atom

Th
ShellElectrons
1 (K) 2
2 (L) 8
3 (M) 18
4 (N) 32
5 (O) 18
6 (P) 10
7 (Q) outermost 2
Total90

Configuration [Rn] 6d² 7s²

6d ↑ ↑
7s ↑ ↓
A shell diagram: each ring is an electron shell and each dot is one electron. The nucleus holds 90 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 Rn core, with one arrow per electron.

Related elements

Actinides
Ac Actinium Pa Protactinium U Uranium Np Neptunium Pu Plutonium Am Americium Cm Curium Bk Berkelium Cf Californium Es Einsteinium Fm Fermium Md Mendelevium No Nobelium Lr Lawrencium

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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), 2019. Toxicological Profile for Thorium, Chapter 1: Relevance to Public Health , Chapter 1, overview.

    • concentrations of thorium in air, food, and/or drinking water are normally very low and thorium-containing substances are not generally readily absorbed by the body.
    • Hematological effects and pulmonary effects (cirrhosis and cancer of the lung) were observed in animals following inhalation exposure to thorium.
  4. 4

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

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

    • Thorium has no known biological role. It is toxic due to its radioactivity.
  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.