What oxidation states can oxygen have?

Oxygen can have oxidation states of −2, −1, 0, +1, and +2. The most common is −2, found in water, most metal oxides, and organic compounds. The −1 state appears in peroxides (H₂O₂, Na₂O₂). Zero occurs in elemental oxygen (O₂) and ozone (O₃). The rare positive states +1 and +2 occur only when oxygen bonds to fluorine, the only element more electronegative than oxygen.

Why is oxygen almost always −2?

Oxygen has an electronegativity of 3.44 (Pauling scale), second only to fluorine (3.98). In bonds with nearly all other elements, oxygen attracts electron density toward itself, formally gaining two electrons to reach a filled 2p⁶ configuration (isoelectronic with neon). This −2 state is so thermodynamically stable that exceptions — peroxides, superoxides, fluorides — require special conditions to form or persist.

What is the oxidation state of oxygen in water (H₂O)?

In H₂O, each hydrogen is +1 and the molecule is neutral: O + 2(+1) = 0, so O = −2. This is the standard −2 assignment that applies to water and nearly all oxygen-containing compounds except peroxides and fluorides.

What is the oxidation state of oxygen in hydrogen peroxide (H₂O₂)?

In H₂O₂, hydrogen is +1 and the molecule is neutral: 2(+1) + 2O = 0, so O = −1. The O–O single bond means oxygen does not formally gain the second electron it would need to reach −2. Peroxides are defined by this −1 oxygen state.

Can oxygen have a positive oxidation state?

Yes, but only in compounds with fluorine. In oxygen difluoride (OF₂), F = −1 (most electronegative element), so O + 2(−1) = 0 gives O = +2. In dioxygen difluoride (O₂F₂), O = +1. These are the only known cases of positive oxygen oxidation states because fluorine is the sole element that out-competes oxygen for electron density.

What is the oxidation state of oxygen in ozone (O₃)?

In ozone (O₃), all three oxygen atoms are equivalent and the molecule is elemental, so the oxidation state is 0 by convention — exactly the same as in O₂. Ozone is not a peroxide and has no O–O single bond in the usual sense; it has a resonance structure with partial double-bond character throughout.

How does the −1 state in superoxides differ from peroxides?

Both peroxides (O₂²⁻) and superoxides (O₂⁻) contain oxygen at −1. In peroxides the O–O bond order is 1 (single bond). In superoxides the O₂⁻ radical anion has a bond order of 1.5, and each oxygen is formally −½ on average — but is still conventionally assigned −1. Superoxides form with the heavier alkali metals (K, Rb, Cs) due to crystal packing effects.

Oxygen Oxidation States

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

Reference tool covering all oxidation states of oxygen from −2 to +2, with compound examples, electron configuration, assignment rules, and an interactive step-by-step finder.

Atomic # 8 O Oxygen

Atomic Mass

15.999 u

Group

16 (VIA)

Period

Block

p-block

Electronegativity

3.44 (Pauling)

Oxidation States

−2, −1, 0, +1, +2

Oxygen has the ground-state configuration [He] 2s² 2p⁴, giving it six valence electrons and two vacancies in the 2p subshell. Its electronegativity of 3.44 — second only to fluorine — means oxygen almost always draws electron density toward itself, sitting at −2 in the vast majority of compounds. The five distinct oxidation states range from −2 (fully oxidized partners) to +2 (rare fluorine compounds where fluorine's superior electronegativity reverses the usual pattern).

State	Stability	Example	Notes
−2	Dominant — extremely common	H₂O, CaO, CO₂, SO₄²⁻	Oxygen formally gains 2 electrons, reaching the neon configuration (2s² 2p⁶). Found in water, metal oxides, acids, salts, and virtually all organic oxygen-containing compounds. Accounts for over 99% of oxygen chemistry.
−1	Stable in peroxides	H₂O₂, Na₂O₂, BaO₂	The O–O single bond in peroxides means each oxygen shares one bonding pair with another O, leaving only a formal single-electron gain. Hydrogen peroxide is the most familiar example; sodium peroxide (Na₂O₂) and barium peroxide (BaO₂) are inorganic peroxides used as bleaches and oxidants.
0	Stable (elemental oxygen)	O₂, O₃	Elemental oxygen has oxidation state 0 by convention — both atoms are identical, so no electron transfer is defined. Dioxygen (O₂) is the standard state; ozone (O₃) is the allotrope with bond order ~1.5. Molecular oxygen is the terminal electron acceptor in aerobic respiration.
+1	Rare — only with fluorine	O₂F₂ (FOOF)	Dioxygen difluoride is a highly reactive, thermally unstable orange-yellow solid. Fluorine (electronegativity 3.98) is the only element that oxidizes oxygen, pulling electron density away to give O a formal +1 state. Forms at low temperature; decomposes rapidly above −57 °C.
+2	Very rare — only with fluorine	OF₂ (oxygen difluoride)	The highest known oxidation state of oxygen. In OF₂, each fluorine is −1 and the neutral molecule gives O = +2. A colorless, toxic gas produced by the reaction of F₂ with dilute NaOH. Used as an oxidizing agent in rocket propellants. Reacts explosively with water.

Assignment Rules for Oxygen
1. Elemental oxygen (O₂, O₃) = 0. 2. In peroxides (O–O bond present) = −1. 3. In superoxides (O₂⁻ radical) = −1 (formally −½ per atom, assigned −1 by convention). 4. In fluorides (OF₂, O₂F₂) = +2 or +1 respectively (F always −1). 5. In all other compounds = −2.
These five rules cover every known oxygen compound. Rule 4 is the only case where oxygen is positive.

The table below covers key inorganic and organic molecules containing oxygen. The derivation column shows the arithmetic used to assign the oxidation state step by step.

Compound	Formula	O State	How to derive it
Water	H₂O	−2	O + 2(+1) = 0 → O = −2
Carbon Dioxide	CO₂	−2	O + C + O = 0; C = +4 → O = −2
Sulfate ion	SO₄²⁻	−2	S + 4O = −2; S = +6 → O = −2
Calcium Oxide	CaO	−2	Ca = +2 (ionic); O = −2
Nitric Acid	HNO₃	−2	H = +1, N = +5; O = −2 (3 oxygens: −6 total)
Ethanol	C₂H₅OH	−2	O bonded to H (+1) and C. O + (+1) + C = group sum → O = −2
Hydrogen Peroxide	H₂O₂	−1	2(+1) + 2O = 0 → O = −1 (O–O bond, peroxide)
Sodium Peroxide	Na₂O₂	−1	Na = +1; 2(+1) + 2O = 0 → O = −1
Barium Peroxide	BaO₂	−1	Ba = +2; (+2) + 2O = 0 → O = −1
Potassium Superoxide	KO₂	−1	K = +1; (+1) + O₂⁻ = 0 → O₂⁻ total −1, each O = −½ (conventionally −1)
Molecular Oxygen	O₂	0	Elemental form — oxidation state 0 by convention.
Ozone	O₃	0	Elemental allotrope — all O atoms identical, state = 0.
Dioxygen Difluoride	O₂F₂	+1	F = −1; 2O + 2(−1) = 0 → O = +1 each
Oxygen Difluoride	OF₂	+2	F = −1; O + 2(−1) = 0 → O = +2

Ground State Configuration

Core (noble gas He)

Valence electrons

Four electrons: two paired, two unpaired (Hund's rule)

Notation

Full: 1s² 2s² 2p⁴

Noble gas: [He] 2s² 2p⁴

O²⁻ (oxide): 1s² 2s² 2p⁶ (= Ne core)

O⁻ (peroxide): 1s² 2s² 2p⁵

Ionization & Electron Gain Steps

[He] 2s² 2p⁴

Neutral atom (8 electrons)

O⁻

[He] 2s² 2p⁵

Gain one electron (EA₁ = −141 kJ/mol — exothermic)

O²⁻

[He] 2s² 2p⁶

Gain second electron (EA₂ = +780 kJ/mol — endothermic; stabilized by lattice energy in ionic solids)

O⁺

[He] 2s² 2p³

Remove one 2p electron — IE₁ 1313.9 kJ/mol

O²⁺

[He] 2s² 2p²

Remove second 2p electron — IE₂ 3388.3 kJ/mol

Why oxygen's 2p⁴ configuration matters
With four 2p electrons, oxygen needs only two more to complete the subshell (2p⁶). This strong drive to gain two electrons explains the dominance of the −2 state. The first electron affinity (−141 kJ/mol) is favorable, but the second (+780 kJ/mol) is endothermic — the O²⁻ ion only exists in solids where lattice energy compensates. In covalent bonds, oxygen reaches the effective −2 state via shared pairs rather than ionic transfer.

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Summary

Reference tool covering all oxidation states of oxygen from −2 to +2, with compound examples, electron configuration, assignment rules, and an interactive step-by-step finder.

How it works

Select a tab — Oxidation States, Compounds, Electron Config, or Identifier — to explore each topic.
The atom card shows oxygen's core data: atomic number, mass, group, electronegativity, and all oxidation states.
The Oxidation States tab lists every state from −2 to +2 with stability notes, examples, and chemical context.
The Compounds tab provides a table of common molecules with their oxygen oxidation state and a derivation.
The Electron Config tab walks through orbital filling and how electron gain or loss changes the configuration.
The Identifier tab lets you choose a molecule and see its oxygen oxidation state derived step by step.

Use cases

Students learning oxidation state assignment rules in general or inorganic chemistry.
Chemistry teachers explaining why peroxides differ from regular oxides.
Anyone studying for A-level, AP Chemistry, or university general chemistry exams.
Researchers checking oxygen oxidation states in reaction mechanisms or redox balancing.
Lab chemists working with peroxides, superoxides, or fluorine-oxygen compounds.

Frequently Asked Questions

Last updated: 2026-06-18 · Reviewed by Nham Vu