Chemistry 2e · Representative Metals, Metalloids, and Nonmetals
Occurrence, Preparation, and Compounds of Oxygen
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In 30 seconds
Oxygen is the most abundant element in Earth's crust (about 46% by mass) and the second most abundant in the atmosphere (about 21% by volume), and it is the element life depends on for respiration. In the air it exists as O2, a colorless, odorless diatomic gas; high in the stratosphere it also forms ozone, O3, which shields living things from ultraviolet radiation. Almost everywhere else, oxygen is combined with other elements as oxides — water (H2O), sand (SiO2), rust (Fe2O3), limestone (CaCO3) — because it is the most electronegative element after fluorine and reacts with nearly everything given enough energy.
The chemistry of oxygen is the chemistry of the oxygen atom's hunger for electrons. In compounds it usually takes an oxidation state The charge an atom would have if electrons were assigned to the more electronegative atom of -2 (oxides), but it also forms peroxides (-1, as in H2O2) and superoxides (-1/2, as in KO2). Elemental O2 is paramagnetic Attracted into a magnetic field because of unpaired electrons Full entry → — attracted into a magnetic field because it has two unpaired electrons — a fact molecular orbital theory explains beautifully. combustion Rapid reaction with oxygen that releases heat and light Full entry →, rusting, respiration, and the ozone layer are all the same theme: oxygen accepting electrons.
Why this matters
Oxygen sustains aerobic life: our cells use it as the final electron acceptor in the electron transport chain, and without it ATP production collapses within minutes. Clinically, supplemental oxygen is among the most common treatments in medicine, and blood oxygen saturation is a routine vital sign. Industrially, oxygen is used in steelmaking, welding torches, wastewater treatment, and rocket propulsion. Ozone matters in two opposite ways: stratospheric ozone protects us from UV-B (the ozone hole and the CFC ban are landmark stories), while ground-level ozone is a harmful smog component that irritates lungs. Fire safety also hinges on oxygen: combustion needs fuel, heat, and O2, which is why removing oxygen (smothering) or cooling (water) extinguishes fires, and why oxygen-enriched environments are severe fire hazards.
The college version
Core Concepts
Occurrence: air, water, and rock
Oxygen is everywhere: O2 makes up about 21% of dry air by volume; water is 89% oxygen by mass; silicates and carbonates dominate the crust. The oxygen cycle connects photosynthesis (which produces O2 from CO2 and H2O) with respiration and combustion (which consume it). The atmosphere's oxygen is essentially a biological product: almost all of it was generated by photosynthetic organisms over billions of years.
Preparation: from air, from water, from compounds
- From air: Air is liquefied and fractionally distilled; nitrogen (bp -196 °C) boils off first, then oxygen (bp -183 °C), typically 99%+ O2.
- From water: electrolysis Using electric current to drive a nonspontaneous reaction Full entry → splits water: 2H2O(l) → 2H2(g) + O2(g), giving very pure gases at the cost of electrical energy.
- From compounds: Heating or catalyzing oxygen-rich compounds releases O2 — hydrogen peroxide Compound containing O22-, with oxygen at -1 Full entry → decomposes, 2H2O2 → 2H2O + O2, and potassium chlorate gives 2KClO3 → 2KCl + 3O2. These reactions produce a gas that supports combustion, so lab use requires proper ventilation and heat management as a general safety principle.
The key industrial fact: almost all commercial oxygen comes from liquefying air, not from chemical reactions.
Properties of O2: paramagnetism and reactivity
Liquid oxygen is pale blue and attracted by a magnet — O2 is paramagnetic because molecular orbital theory places two unpaired electrons in antibonding π* orbitals. A simple Lewis structure with all electrons paired cannot explain this. Chemically, O2 is a powerful oxidizing agent: it supports combustion, reacts with most metals and nonmetals to form oxides, and its reactivity increases with temperature. Pure oxygen makes fires burn dramatically faster than air does — a fundamental safety fact for anyone handling oxygen cylinders or enriched atmospheres.
Ozone: the same element, a different molecule
Ozone, O3, is an allotrope A different structural form of the same element of oxygen: a bent molecule with a bond order of 1.5 between each pair of oxygens (resonance). In the stratosphere, O3 absorbs most UV-B radiation, protecting DNA from damage. Chlorofluorocarbons (CFCs) release chlorine atoms that catalytically destroy ozone — one Cl atom can destroy many O3 molecules — producing the seasonal ozone hole over Antarctica and leading to the Montreal Protocol ban on CFCs. At ground level, ozone is a pollutant: it forms when sunlight drives reactions of NOx and volatile organics from vehicles, and it damages lungs and crops. Same element, completely different roles depending on altitude.
Oxides and the oxidation states of oxygen
Oxygen's common oxidation states are -2 (oxides: H2O, CO2, Na2O), -1 (peroxides: H2O2, Na2O2), and -1/2 (superoxides: KO2, used in breathing masks because it releases O2 on contact with water and CO2). Only with fluorine does oxygen take a positive state — in OF2, oxygen is +2. Oxides can be classified by acid–base behavior: basic oxides (metal oxides like Na2O and CaO, which react with acids), acidic oxides (nonmetal oxides like CO2, SO2, and P4O10, which react with bases and water to form acids), amphoteric oxides (like Al2O3, which react with both acids and bases), and neutral oxides (like CO and NO).
How It Works / Step-by-Step Process
How a hospital gets oxygen: (1) Air is compressed and cooled through repeated compression–expansion cycles until it liquefies; (2) the liquid air is warmed in a distillation column, where N2 leaves first, then argon, then O2; (3) the oxygen (typically ≥99.5%) is piped as gas or chilled into liquid oxygen for transport in insulated tanks; (4) at the point of use it is vaporized and delivered through flow meters. The chain is built around one safety principle: oxygen strongly supports combustion, so equipment is kept clean, oil-free, and away from ignition sources.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Ozone being "good" | Ozone being universally good | Stratospheric ozone protects from UV; ground-level ozone is a toxic pollutant — altitude decides. |
| O2 as a "paired-electron" molecule | O2's actual electron structure | O2 has two unpaired electrons (paramagnetic); fully paired Lewis structures contradict the magnetic evidence. |
| Oxygen being most electronegative | Fluorine actually is | Fluorine is more electronegative; only in OF2 does oxygen carry a positive state. |
| Peroxides and oxides | The same thing | Peroxides contain O22- (O at -1); oxides contain O2- (O at -2). H2O2 vs H2O is the classic pair. |
| "Oxygen supports combustion" | "Oxygen burns" | Oxygen itself does not burn; it feeds the burning of other materials — pure O2 makes fires far more intense. |
| All oxides being acidic | Acid–base behavior of oxides | Metal oxides are basic, nonmetal oxides acidic, Al2O3/ZnO amphoteric, CO/NO neutral. |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Oxygen is the element that makes fire burn and lets you breathe — it's like the "food" that both flames and your body eat. It loves grabbing electrons from other atoms, which is why iron rusts and apples turn brown. High up, a different kind of oxygen called ozone acts like a sunscreen for the whole planet, while near the ground the same molecule is a troublemaker that stings your lungs.
Worked example
Worked Example 1 — Oxygen from hydrogen peroxide. Hydrogen peroxide decomposes as 2H2O2 → 2H2O + O2. What mass of O2 is released from 68.0 g of H2O2? Molar masses: H2O2 = 2(1.008) + 2(16.00) = 34.02 g/mol; O2 = 32.00 g/mol. Conversion path first, then substitute:
g O2 = 68.0 g H2O2 × 1 mol H2O234.02 g H2O2 × 1 mol O22 mol H2O2 × 32.00 g O21 mol O2 = 32.0 g O2
Units cancel: g H2O2 → mol H2O2 → mol O2 → g O2. So 68.0 g of hydrogen peroxide gives 32.0 g of oxygen gas.
Worked Example 2 — Percent oxygen by mass in a compound. Calculate the percent oxygen in water, H2O. Molar mass of H2O = 2(1.008) + 16.00 = 18.02 g/mol; oxygen contributes 16.00 g/mol. The percent by mass is:
%O = 16.00 g O18.02 g H2O × 100% = 88.8%
So water is about 89% oxygen by mass — a useful check when converting between masses of water and oxygen in stoichiometry.
Key takeaways
- Oxygen: ~21% of air by volume, ~46% of Earth's crust by mass.
- Commercial O2 comes mainly from fractional distillation of liquid air (bp O2 = -183 °C, above N2's -196 °C).
- O2 is paramagnetic (two unpaired π* electrons) — a classic MO result.
- Ozone O3 is an allotrope; stratospheric O3 blocks UV-B; ground-level O3 is a pollutant; CFC-derived chlorine destroys stratospheric ozone.
- Oxidation states: -2 oxides, -1 peroxides, -1/2 superoxides, +2 only in OF2.
- Oxides: basic (metal), acidic (nonmetal), amphoteric (Al2O3), neutral (CO, NO).
- Combustion needs fuel + O2 + heat; removing any one stops the fire.
Check yourself
5 review questions from the chapter. Try each one, then open the answer.
How is commercial oxygen produced, and why does nitrogen come off first?
Show answer
By fractional distillation of liquid air; nitrogen boils at -196 °C, below oxygen's -183 °C, so N2 vaporizes first.
Why is O2 paramagnetic, and what does that reveal about its electron structure?
Show answer
Molecular orbital theory places two unpaired electrons in the π* antibonding orbitals of O2, making it attracted to a magnetic field.
Write the balanced decompositions of hydrogen peroxide and potassium chlorate.
Show answer
2H2O2 → 2H2O + O2; 2KClO3 → 2KCl + 3O2.
What are the oxidation states of oxygen in H2O, H2O2, and KO2?
Show answer
H2O: -2; H2O2: -1; KO2: -1/2.
What mass of O2 forms when 122.6 g of KClO3 decomposes? (Molar masses: KClO3 122.55, O2 32.00 g/mol; reaction: 2KClO3 → 2KCl + 3O2.)
Show answer
122.6 g KClO3 × (1 mol/122.55 g) × (3 mol O2/2 mol KClO3) × (32.00 g/1 mol) = 48.0 g O2.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- oxidation state
- The charge an atom would have if electrons were assigned to the more electronegative atom
- allotrope
- A different structural form of the same element
- paramagnetic
- Attracted into a magnetic field because of unpaired electrons
- combustion
- Rapid reaction with oxygen that releases heat and light
- peroxide
- Compound containing O22-, with oxygen at -1
- superoxide
- Compound containing O2-, with oxygen at -1/2
- amphoteric oxide
- An oxide that reacts with both acids and bases
- electrolysis
- Using electric current to drive a nonspontaneous reaction
Sources & references
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