Selenium

Element 34 · Nonmetal

Specimen

Atomic number: 34
Atomic mass: 78.97
State at room temperature: Solid
Melting point: 220.5 °C
Boiling point: 684.9 °C
Density: 4.809 g/cm³
Discovered: 1817

Physical data from PubChem.¹

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. ²³

The atom

Shell	Electrons
1 (K)	2
2 (L)	8
3 (M)	18
4 (N) outermost	6
Total	34

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

Sources

1

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

Atomic mass, electron configuration, melting and boiling points, density and discovery year for all 118 elements.
2
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
3
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

Specimen

Can I eat it?

The atom

Related elements

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Sources