What are selenium's common oxidation states?

Selenium most commonly appears as -2 (selenide), +4 (selenious compounds), and +6 (selenic compounds). The +2 state is rare and found only in a few specialty compounds. The 0 state is elemental selenium.

Why can selenium reach a +6 oxidation state?

Selenium is in Period 4, so it has access to empty 3d orbitals. This allows it to expand its valence shell beyond 8 electrons and accommodate up to 6 bonds, enabling the +6 state seen in H2SeO4 (selenic acid).

What is the difference between selenite and selenate?

Selenite contains Se in the +4 state (SeO3²⁻ ion, as in Na2SeO3), while selenate contains Se in the +6 state (SeO4²⁻ ion, as in Na2SeO4). Selenate is the more oxidized, more stable form at higher oxidation potentials.

Yes — H2Se (hydrogen selenide) is the selenium analog of hydrogen sulfide (H2S). In both, the chalcogen is in the -2 oxidation state. H2Se is a more toxic, stronger acid, and has weaker O–H bond analogs due to the larger Se atom.

What is selenium's electron configuration?

Ground state: [Ar] 3d10 4s2 4p4. Selenium has 6 valence electrons (4s2 4p4). The -2 state fills the 4p shell to 4p6 (noble-gas-like), while +4 and +6 involve bonding via the 4p and 4d orbitals respectively.

Is selenium essential to human health?

Yes, selenium is an essential trace element. It is incorporated into selenoproteins such as glutathione peroxidase, which protects cells from oxidative damage. Both deficiency and excess (selenosis) are harmful; dietary intake is tightly regulated.

Selenium Oxidation States

Name: Selenium Oxidation States
Availability: InStock
Author: Nham Vu

Reference for selenium oxidation states (-2, +2, +4, +6), electron configurations, common compounds, and redox behavior.

Atomic # 34 Se Selenium

Atomic Mass

78.971 u

Group

16 (VIA)

Period

Block

p-block

Electronegativity

2.55 (Pauling)

Oxidation States

-2, +2, +4, +6

Selenium (Group 16, Period 4) has the ground-state valence configuration 4s² 4p⁴, giving it 6 valence electrons. This allows four significant oxidation states. The -2 state (selenide) is the most reduced and arises when selenium gains 2 electrons to fill its 4p shell. The +4 and +6 states involve progressive removal of 4p and 4s electrons in bonding to electronegative atoms like oxygen and fluorine, with Period-4 d-orbital participation enabling the +6 maximum.

State	Stability	Example	Notes
-2	Stable	H₂Se, Na₂Se	Se fills its 4p shell (4p6) — isoelectronic with Kr core. Most reduced form; found in selenide salts and hydrogen selenide.
0	Elemental	Se (gray/red)	Pure selenium; assigned 0 by convention. Exists as Se8 rings (red) or infinite helical chains (gray).
+2	Rare	Se₂Cl₂	Uncommon; found in a few dihalide and polychalcogenide species. Not observed in aqueous solution.
+4	Stable	SeO₂, H₂SeO₃	Common in oxide and oxyacid chemistry. SeO2 forms on burning Se in air. Selenite ion (SeO3²⁻).
+6	Stable (ox.)	H₂SeO₄, SeF₆	Most oxidized state; strong oxidizing agent. Selenic acid (H2SeO4) is analogous to sulfuric acid. Selenate ion (SeO4²⁻).

Redox Ladder (most reduced → most oxidized)

-2 Se²⁻ → 0 Se → +4 Se(IV) → +6 Se(VI)

Standard reduction potential Se(VI)/Se(IV): +1.15 V | Se(IV)/Se(0): +0.74 V | Se(0)/Se(-II): -0.11 V

Selenium forms compounds across all four oxidation states, though -2, +4, and +6 are by far the most common in practice. Each state gives selenium a distinct chemical character — from nucleophilic selenide anions (-2) to powerful oxidizing selenate salts (+6).

Compound	Formula	Se State	Notes
Hydrogen Selenide	H₂Se	-2	Colorless, very toxic gas; analog of H2S; stronger acid; pungent odor.
Sodium Selenide	Na₂Se	-2	Ionic selenide salt; used in semiconductor and glass manufacturing.
Zinc Selenide	ZnSe	-2	Wide-bandgap semiconductor; used in LEDs and laser diodes (blue-green).
Lead Selenide	PbSe	-2	Narrow-gap semiconductor used in infrared detectors and thermoelectrics.
Selenium Dioxide	SeO₂	+4	White solid; forms when Se burns in air; oxidizing agent in organic synthesis.
Selenious Acid	H₂SeO₃	+4	Weak diprotic acid; used in metal finishing and glass coloring.
Sodium Selenite	Na₂SeO₃	+4	Water-soluble selenite salt; dietary supplement source of selenium.
Selenium Tetrafluoride	SeF₄	+4	Reactive fluoride; tetrahedral with lone pair; hydrolyzes in water.
Selenic Acid	H₂SeO₄	+6	Strong diprotic acid analogous to H2SO4; powerful oxidizing agent.
Sodium Selenate	Na₂SeO₄	+6	Isomorphous with sodium sulfate; used in agriculture and as a tracer.
Selenium Hexafluoride	SeF₆	+6	Colorless gas; octahedral; kinetically inert but toxic; used as a dielectric.
Dimethyl Selenide	(CH₃)₂Se	-2	Volatile organoselenide; garlic odor; metabolite of selenium in organisms.

Ground State Configuration

Core (He)

Core

Core (Ar-like core up to here)

Core (filled d-shell)

Valence

4 electrons in 4p (4 of 6 slots)

Notation

Full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁴

Noble gas: [Ar] 3d¹⁰ 4s² 4p⁴

How Each State Arises

-2

Gains 2e⁻ → 4p⁶ (Kr-like core)

Neutral; 4s² 4p⁴ unchanged

Loses 2×4p electrons (rare bonding mode)

4p lone pair donates into 2 bonds + lone pair; loses 4p² effectively in bonding

All 4p + 4s electrons involved in bonding; 3d participation enables expanded octet

d-orbital expansion: Unlike oxygen (Period 2, no 2d orbitals), selenium has accessible 3d orbitals. This allows it to form 6 bonds in SeF₆ or H₂SeO₄, which is impossible for oxygen or sulfur's lighter analogs at the same geometry.

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Summary