Oxygen (O)

Liquid oxygen is pale blue, and it is paramagnetic. Between the poles of a strong magnet it does not fall away: it bridges the gap and stays there until it boils off. Nothing in the bonding diagram taught in a first chemistry course predicts that, and for a while nobody could explain it at all.

The molecule that embarrassed Lewis structures

Drawn the conventional way, O₂ has a double bond and two lone pairs on each atom. Every electron is paired off. A molecule with all its electrons paired is diamagnetic — it is weakly repelled by a magnetic field, not attracted to it.

Oxygen is strongly attracted, which means it has unpaired electrons. Two of them, in fact. The dot-and-cross picture is not merely incomplete here; it gives the wrong answer about an observable property.

Molecular orbital theory produces the right answer. When the atomic orbitals of two oxygen atoms combine, the resulting π* antibonding orbitals come out degenerate — two orbitals at the same energy — and the last two electrons occupy one each with parallel spins rather than pairing up in one. The molecule is a ground-state triplet. This is the standard argument for teaching molecular orbitals at all, and oxygen is the reason it wins.

The triplet ground state has a consequence far larger than a lecture demonstration. Most organic molecules are singlets, with all electrons paired, and a direct reaction between a singlet and a triplet is spin-forbidden. That kinetic barrier is the only reason a forest, or a person, does not spontaneously combust in air. Oxygen is thermodynamically desperate to oxidise almost everything around it and is prevented from doing so at room temperature by a quantum-mechanical technicality.

Three men found it and the one who understood it named it wrong

The priority question here is not a matter of shading — it is a genuine three-way dispute, and the single credit on the data card above cannot carry it.

Carl Wilhelm Scheele, working in a Swedish pharmacy, prepared the gas some time around 1771–72, called it "fire air", and wrote it up. His book was delayed by a publisher and a promised preface and did not appear until 1777. He was first to the substance and last into print.

Joseph Priestley isolated it on 1 August 1774 by focusing sunlight onto mercuric oxide, and published in 1775. He interpreted it within phlogiston theory as "dephlogisticated air" — air that had been emptied of phlogiston and could therefore absorb more of it from a burning candle. He died still believing in phlogiston.

Antoine Lavoisier met Priestley in Paris in October 1774, heard the details over dinner, and proceeded to build the entire modern theory of combustion on them — that burning is combination with a component of the air, not the release of a substance from the fuel. He also gave the element its permanent name in 1777: oxygène, the acid-former, because he believed every acid contained it.

He was wrong. In 1810 Humphry Davy demonstrated that muriatic acid — hydrochloric acid — contains no oxygen whatsoever. By then the name was in every European language and could not be dislodged. Oxygen is a fossil of a theory that was discarded within a generation of its coinage.

The pollution event that made animals possible

For roughly the first two billion years of Earth's history there was essentially no free oxygen in the atmosphere. Cyanobacteria began releasing it as a waste product of photosynthesis, and the oceans absorbed it chemically for a very long time before any of it reached the air — dissolved ferrous iron oxidised and precipitated out of solution across the world's shallow seas.

Once those sinks were saturated, atmospheric oxygen rose sharply, some 2.4 billion years back. To the anaerobic organisms that then constituted all life, this was a mass poisoning; it is sometimes called the oxygen catastrophe with no irony intended. What survived either retreated to anoxic refuges or evolved to use the new oxidant, and aerobic respiration extracts something like fifteen times more energy per unit of sugar than fermentation does. Every organism large enough to see is downstream of that trade.

Concentrations have not been steady since. Fossil charcoal and the size of Carboniferous arthropods both point to a late Palaeozoic peak well above today's level — the dragonfly relative Meganeura, with a wingspan approaching seventy centimetres, is usually read as evidence that insect tracheal breathing was working in a richer atmosphere.

The same molecule, shielding above and poisoning below

Three oxygen atoms instead of two gives ozone, and ozone's reputation depends entirely on altitude. In the stratosphere it absorbs ultraviolet in the band that damages DNA, in a continuous cycle of photolysis and reformation first described by Sydney Chapman in 1930. At ground level it is an aggressive respiratory irritant and a regulated pollutant, formed when sunlight acts on vehicle exhaust.

The Antarctic ozone hole, identified in 1985 by Joe Farman, Brian Gardiner and Jonathan Shanklin from British Antarctic Survey records, was so far outside expectation that satellite data processing had been discarding the low readings as instrument error. The Montreal Protocol followed in 1987 and is the only United Nations treaty ever ratified by every member state.

Ice cores keep a thermometer in the ratio

Oxygen-18 is heavier than oxygen-16 by about 12%, and that difference is enough to bias evaporation. Water containing the lighter isotope evaporates slightly more readily and the heavier one rains out slightly sooner, so the isotopic composition of precipitation depends on the temperature at which it formed.

The ratio, written as δ¹⁸O, is therefore a thermometer that runs backwards through time. Measured in Greenland and Antarctic ice cores it gives air temperature at the moment the snow fell; measured in the calcite shells of marine foraminifera it gives a combined signal of ocean temperature and global ice volume. The numbered Marine Isotope Stages that structure the whole Quaternary ice-age chronology are defined from those foraminiferal ratios.

Steel takes more of it than hospitals ever will

Ask what industrial oxygen is for and most people say breathing. Medical oxygen is real but small. The dominant consumer, by a wide margin, is steelmaking.

In the basic oxygen furnace, a water-cooled lance blows a jet of nearly pure oxygen into molten pig iron. The carbon dissolved in the iron burns out as carbon monoxide, and the reaction is exothermic enough to need no external heat at all. The process converts a charge of iron into steel in well under an hour, where the Bessemer converter it replaced was slower and the open hearth slower still. Chemical manufacture — ethylene oxide, titanium dioxide, syngas — and rocket oxidiser take much of the remainder.

Nearly all of it comes from the same place: fractional distillation of liquefied air, which separates oxygen from nitrogen on a boiling-point difference of about thirteen degrees.

The atmosphere is very slowly running down

Ralph Keeling — son of Charles Keeling, whose carbon dioxide record is the famous sawtooth curve — developed an interferometric method precise enough to measure changes in atmospheric oxygen against a background of nearly 21%. It is falling, by a few parts per million of the total each year, as fossil carbon combines with it.

The absolute decline is far too small to matter physiologically and will remain so. Its value is forensic: the rate at which oxygen disappears, compared with the rate at which carbon dioxide appears, distinguishes fossil fuel combustion from deforestation and from ocean outgassing. The oxygen budget is one of the cleanest independent confirmations of where the extra carbon in the air has come from.