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Selenium

Element 34 · Nonmetal

Specimen

Atomic number
34
Atomic mass
78.971
State at room temperature
Solid
Melting point
220.5 °C
Boiling point
684.9 °C
Density
4.809 g/cm³
Discovered
1817

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

Can I eat it?

In food and water
Your body needs it. Selenium is an essential nutrient. In 2023, EFSA set an upper limit of 255 µg a day for adults. People who often eat Brazil nuts or take high-dose supplements can go above it. 34

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The atom

Se
ShellElectrons
1 (K) 2
2 (L) 8
3 (M) 18
4 (N) outermost 6
Total34

Configuration [Ar] 3d¹⁰ 4s² 4p⁴

3d ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓
4s ↑ ↓
4p ↑ ↓ ↑ ↑
A shell diagram: each ring is an electron shell and each dot is one electron. The nucleus holds 34 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 Ar core, with one arrow per electron.

Related elements

Group 16
O Oxygen S Sulfur Te Tellurium Po Polonium Lv Livermorium

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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

    European Food Safety Authority (EFSA), 2017. Dietary Reference Values for nutrients. Summary report (amended 2019) .

    Summaries of the EFSA NDA Panel opinions on each vitamin and mineral, section 4 (minerals) and section 5 (cobalamin).

    • Selenium deficiency affects the expression and function of selenoproteins
  4. 4

    EFSA Panel on Nutrition, Novel Foods and Food Allergens (NDA), EFSA Journal 21(1):7704, 2023. Scientific opinion on the tolerable upper intake level for selenium , Abstract.

    • A UL of 255 μg/day is established for adult men and women
    • adult consumers are unlikely to exceed the UL, except for regular users of food supplements containing high daily doses of selenium or regular consumers of Brazil nuts
  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.