What is the atomic number of selenium?

Selenium has atomic number 34, meaning its nucleus contains 34 protons. It is the 34th element in the periodic table, placed in period 4, group 16 (chalcogens).

What is the electron configuration of selenium?

The full electron configuration is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p4, commonly abbreviated as [Ar] 3d10 4s2 4p4. The four 4p electrons explain its typical oxidation states of -2, +2, +4, and +6.

Why is selenium important biologically?

Selenium is an essential trace element incorporated into selenoproteins (e.g., glutathione peroxidase) that protect cells from oxidative damage. Both deficiency and excess are harmful; the safe dietary range is narrow (55–400 µg/day for adults).

What are the common oxidation states of selenium?

Selenium commonly exhibits -2 (selenides, like H2Se), +4 (selenium dioxide, SeO2), and +6 (selenic acid, H2SeO4). The +2 state is rarer but observed in some selenium halides.

How many stable isotopes does selenium have?

Selenium has six naturally occurring stable isotopes: 74Se (0.89%), 76Se (9.37%), 77Se (7.63%), 78Se (23.77%), 80Se (49.61%), and 82Se (8.73%). 80Se is the most abundant.

What is the electronegativity of selenium?

On the Pauling scale, selenium has an electronegativity of 2.55, between sulfur (2.58) and tellurium (2.10). This moderate value allows it to form both covalent and ionic bonds.

Selenium Element Properties

Look up every key property of Selenium (Se, Z=34) — atomic mass, electron configuration, electronegativity, oxidation states, and more — in one place.

34 Se 78.971

Selenium

Selenium is a nonmetal chalcogen (Group 16, Period 4) with atomic number 34. Discovered in 1817 by Jöns Jacob Berzelius, it is named after the Greek moon goddess Selene. It exists in several allotropic forms and is an essential trace element in human nutrition.

Nonmetal Chalcogen Period 4 Group 16 Solid at STP

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Property	Value	Category

Electron Configuration

Full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁴ | Abbreviated: [Ar] 3d¹⁰ 4s² 4p⁴

Naturally Occurring Isotopes

Isotope	Mass (u)	Abundance	Neutrons	Spin
⁷⁴Se	73.9224759	0.89%	40	0+
⁷⁶Se	75.9192137	9.37%	42	0+
⁷⁷Se	76.9199142	7.63%	43	1/2-
⁷⁸Se	77.9173091	23.77%	44	0+
⁸⁰Se	79.9165218	49.61%	46	0+
⁸²Se	81.9166995	8.73%	48	0+

Summary

Selenium (Se) is a nonmetal in group 16 of the periodic table with atomic number 34 and standard atomic weight 78.971 u. It sits between sulfur and tellurium and shares their chalcogen chemistry, forming compounds in the -2, +2, +4, and +6 oxidation states. This tool surfaces every textbook and IUPAC-reference property of selenium in a single, searchable page.

How it works

Browse the categorized property cards for physical, chemical, electronic, and nuclear data.
Use the search box to filter to a specific property by name.
Click any property row to copy its value to the clipboard.
Filter properties by category using the Physical, Chemical, Electronic, or Nuclear buttons.
Inspect the visual subshell occupancy breakdown in the electron-configuration viewer.

Use cases

Quick reference during chemistry homework or exam review.
Verify selenium data for lab reports and research papers.
Compare selenium with neighboring chalcogens (S, Te, Po).
Check oxidation states and electronegativity for reaction prediction.
Look up isotope data for radiochemistry or nuclear engineering coursework.
Confirm physical constants for material-science or semiconductor calculations.
Understand biological roles of selenium as a trace element.
Prepare for standardized tests that cover periodic-table properties.

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Last updated: 2026-09-30 · Reviewed by Nham Vu