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Chemistry: Atoms First 2e · Representative Metals, Metalloids, and Nonmetals

Occurrence, Preparation, and Compounds of Oxygen

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Reference-values note: atomic masses, boiling points (−195.8 °C N₂, −183.0 °C O₂), crustal abundance (~46% by mass), and STP molar volume (22.4 L/mol) are commonly taught reference values; use the periodic table provided in your course for graded work. Safety note: oxygen supports combustion vigorously — general principles only (keep flames and oils away from concentrated O₂, ventilate), no lab procedures.
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On this page 9 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Check yourself
  8. Study tools
  9. Sources & references

In 30 seconds

Oxygen (O, atomic number 8) is the most abundant element in Earth's crust (about 46% by mass), the second most abundant in the atmosphere (about 21% by volume as O2), and roughly 89% of the mass of water. It is the great connector of chemistry: oxygen combines with nearly every other element, and its compounds — oxides — dominate minerals, rocks, soils, and the chemistry of combustion, respiration, and corrosion. The element exists in two important molecular forms: dioxygen, O2, the colorless gas we breathe, and ozone, O3, the pale-blue gas that shields us from ultraviolet radiation in the stratosphere. This topic covers where oxygen is found, how it is prepared (fractional distillation of liquid air, electrolysis of water, laboratory decompositions), and its compounds organized by oxidation state: −2 in oxides, −1 in peroxides, −½ in superoxides.

Why this matters

Oxygen chemistry explains everyday phenomena: combustion is rapid oxidation (why a fire needs fuel, oxygen, and heat); respiration is the controlled "burning" of glucose; hemoglobin carries oxygen and makes blood red. Corrosion (rusting) is slow oxidation of iron that costs billions of dollars a year. Ozone matters twice: in the stratosphere it absorbs harmful UV ("good" ozone), while at ground level it irritates lungs. And classifying oxides as acidic, basic, or amphoteric is one of the most-tested ideas in general chemistry.

The college version

Core Concepts

Occurrence: everywhere

Oxygen is the most abundant element in Earth's crust and oceans: about 46% of the crust by mass (in silicates, carbonates, and oxides), about 89% of water by mass, and about 21% of the atmosphere by volume as O2. Nearly all free O2 in the air is produced by photosynthesis — the biological reverse of combustion:

6 CO2(g) + 6 H2O(l) light⟶ C6H12O6(aq) + 6 O2(g)

Preparation: three standard routes

Fractional distillation of liquid air is the commercial method: air is liquefied and slowly warmed; N2 (−195.8 °C) boils off first, leaving oxygen (−183.0 °C), about 99.5% pure, for hospitals, steelmaking, and rockets.

Electrolysis of water produces very pure oxygen (with hydrogen) by passing current through water containing an electrolyte:

2 H2O(l) electrolysis⟶ 2 H2(g) + O2(g)

Laboratory decomposition of hydrogen peroxide is the classic demo source; manganese dioxide catalyzes it:

2 H2O2(aq) MnO2⟶ 2 H2O(l) + O2(g)

Properties: an oxidizing gas with a magnetic secret

Dioxygen is colorless, odorless, and slightly soluble in water (how fish breathe). Two properties deserve attention. First, O2 is paramagnetic — attracted into a magnetic field — because molecular orbital theory predicts (and experiment confirms) two unpaired electrons; liquid oxygen visibly pours between the poles of a strong magnet. Second, oxygen is a powerful oxidizing agent: it accepts electrons from almost every element except the noble gases and a few metals (gold and platinum do not react directly with O2 under ordinary conditions) — which is why "burning" is defined as rapid oxidation.

Ozone, O3, is an allotrope of oxygen: a bent, pale-blue gas, a much stronger oxidizing agent than O2, and the absorber of most solar UV-B in the stratosphere. It forms when O2 is subjected to an electric discharge (lightning, ozonizers).

Compounds: the oxidation states of oxygen

Oxygen normally has oxidation state −2, as in water (H2O) and magnesium oxide (MgO). It is −1 in peroxides — compounds with the O22- ion, such as H2O2 and Na2O2 — and −½ in superoxides — containing O2-, such as KO2, used in breathing gear because it releases O2 while absorbing CO2. Only with fluorine, the one element more electronegative than oxygen, does oxygen show a positive oxidation state: +2 in OF2.

Classifying oxides: acidic, basic, amphoteric, neutral

A favorite exam topic with a simple periodic rule:

  • Basic oxides form with metals (e.g., Na2O, CaO) and react with water to give bases: CaO(s) + H2O(l) → Ca(OH)2(aq).
  • Acidic oxides form with nonmetals (e.g., CO2, SO2, NO2) and react with water to give acids: CO2(g) + H2O(l) → H2CO3(aq).
  • Amphoteric oxides (e.g., Al2O3, ZnO) react with both acids and bases.
  • Neutral oxides (e.g., CO, N2O) show neither behavior in water.

Common Confusions

Do Not ConfuseWithDifference
"Oxygen burns""Oxygen supports burning"O₂ is the oxidizer, not the fuel; a flame needs fuel + O₂ + heat
Oxide (O2-, O = −2)Peroxide (O22-, O = −1)Peroxides have an O–O bond and one fewer electron per O; H₂O₂ vs H₂O
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Oxygen is the "sharing superstar" of chemistry — it grabs electrons from almost everything, and when it does, we call it burning or rusting. Plants make it from sunlight and water; we bottle it by cooling air to a liquid and boiling nitrogen away first. Its twin, ozone, up high acts like sunglasses for Earth.

Worked example

Example 1: Combustion stoichiometry — burning magnesium

How many grams of O2 are required to react completely with 48.6 g of magnesium? Balanced equation: 2 Mg(s) + O2(g) → 2 MgO(s). (Molar masses: Mg = 24.31 g/mol, O₂ = 32.00 g/mol.)

Step 1 — Grams to moles:

n(Mg) = 48.6 g × 1 mol Mg24.31 g Mg = 2.00 mol Mg

Step 2 — Mole ratio (1 mol O₂ per 2 mol Mg):

2.00 mol Mg × 1 mol O22 mol Mg = 1.00 mol O2

Step 3 — Moles to grams:

1.00 mol O2 × 32.00 g O21 mol O2 = 32.0 g O2

Unit check: g Mg → mol Mg → mol O₂ → g O₂. Answer: 32.0 g O₂.

Example 2: Gas volume from hydrogen peroxide decomposition

What volume of O2 at STP (22.4 L/mol) is produced when 68.0 g of H2O2 decomposes completely? Balanced equation: 2 H2O2(aq) → 2 H2O(l) + O2(g). (Molar mass H₂O₂ = 34.01 g/mol.)

Step 1 — Grams to moles:

n(H2O2) = 68.0 g × 1 mol H2O234.01 g H2O2 = 2.00 mol H2O2

Step 2 — Mole ratio (1 mol O₂ per 2 mol H₂O₂):

2.00 mol H2O2 × 1 mol O22 mol H2O2 = 1.00 mol O2

Step 3 — Moles to volume at STP:

V = 1.00 mol O2 × 22.4 L1 mol = 22.4 L O2

Check: this is why 3% hydrogen peroxide fizzes on a cut — blood catalase releases the oxygen.

Example 3: Percent oxygen in water

Water is about what percent oxygen by mass? (Molar masses: H = 1.008, O = 16.00 g/mol.)

Step 1 — Formula:

M(H2O) = 2(1.008) + 16.00 = 18.02 g/mol

Step 2 — Percent:

%O = 16.00 g O18.02 g H2O × 100% = 88.8%

That "89%" quoted in textbooks is exactly this — nearly nine-tenths of the mass of every glass of water is oxygen.

Key takeaways

  • Oxygen is the most abundant element in Earth's crust (~46% by mass) and ~89% of water by mass; O2 is ~21% of the atmosphere by volume.
  • Commercial preparation: fractional distillation of liquid air (N₂ boils off at −195.8 °C; O₂ at −183.0 °C).
  • O2 is paramagnetic (two unpaired electrons) — evidence for molecular orbital theory.
  • Oxidation states: −2 (oxides), −1 (peroxides, O22-), −½ (superoxides, O2-), +2 (only in OF2).
  • Oxide classification: metal → basic; nonmetal → acidic; Al₂O₃/ZnO → amphoteric; CO/N₂O → neutral.
  • Ozone O3: stratospheric UV shield (good) vs ground-level pollutant (bad); stronger oxidizer than O2.
  • Photosynthesis: 6 CO2 + 6 H2O → C6H12O6 + 6 O2 — the source of nearly all atmospheric O2.

Check yourself

5 review questions from the chapter. Try each one, then open the answer.

  1. Which element is most abundant in Earth's crust, and roughly what percent is oxygen?

    Show answer

    Oxygen (~46% by mass) is the most abundant element in the crust; silicon is second (~28%).

  2. Why is O2 paramagnetic, and what theory explains it?

    Show answer

    O₂ has two unpaired electrons in antibonding π orbitals; molecular orbital theory predicts and explains this (a Lewis structure cannot).

  3. What are the oxidation states of oxygen in MgO, H2O2, and KO2?

    Show answer

    MgO: −2; H₂O₂: −1 (each O in the O–O bond); KO₂: −½ (each O in O2-).

  4. How many grams of O2 come from decomposing 170.0 g of H2O2 (H₂O₂ = 34.01 g/mol)?

    Show answer

    Moles H₂O₂ = 170.0 g ÷ 34.01 g/mol = 5.00 mol; moles O₂ = 5.00 × 12 = 2.50 mol; mass = 2.50 × 32.00 = 80.0 g O2.

  5. Classify these oxides as acidic, basic, or amphoteric: Na2O, SO2, Al2O3.

    Show answer

    Na2O: basic (metal oxide); SO2: acidic (nonmetal oxide); Al2O3: amphoteric (reacts with both acids and bases).

Keep learning

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Study tools & related lessonsKey vocabulary · Related

Key vocabulary

Dioxygen (O₂)
The diatomic, breathable form of oxygen
Ozone (O₃)
Triatomic allotrope of oxygen

Sources & references

  1. openstax.org — Chemistry Atoms First 2e

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

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