Oxygen

• 1. Introduction

  ›

  Oxygen makes up almost half of the Earth's crust, yet for most of human history nobody knew it existed. It carries the symbol O and the atomic number 8, a member of the chalcogen group in the periodic table. It is the third most abundant element in the entire universe, behind only hydrogen and helium. Every breath an adult takes at rest pulls in between 1.8 and 2.4 grams of it each minute. Across all of humanity, that adds up to more than 6 billion tonnes inhaled per year. This is the story of a colorless, odorless gas that almost no scientist could see clearly even while holding it in a glass tube. How did a substance so essential to life stay hidden for so long? Why did the man who first released it from a heated mineral fail to recognize what he had made? And how does a gas too reactive to linger in the air keep replenishing itself, breath after breath, age after age?

• 2. The Long Misunderstanding

  ›

  Philo of Byzantium, a 2nd-century BCE Greek writer on mechanics, inverted a vessel over a burning candle and watched water rise into its neck. In his work Pneumatica, Philo guessed wrongly that part of the air had turned into the classical element fire and escaped through pores in the glass. Centuries of confusion followed his honest mistake. Ibn al-Nafis, writing in 1250 CE, correctly described how blood is oxygenated in the circulatory system. Michael Servetus rediscovered the same idea in 1553, but his books were systematically destroyed. William Harvey finally published an influential account of the process in 1628. Leonardo da Vinci noticed that combustion and respiration both consume a portion of the air. The Polish alchemist Michael Sendivogius, writing in 1604, came closer than anyone. He described a substance in air he called cibus vitae, or food of life, and recognized it as the gas released when potassium nitrate is heated. Contemporary scientists like Robert Boyle never grasped the connection he had found.

• 3. The Phlogiston Trap

  ›

  Phlogiston theory was established in 1667 by the German alchemist J. J. Becher and modified by Georg Ernst Stahl by 1731. It held that every combustible material contained two parts. One part, phlogiston, was supposedly given off during burning, leaving behind the substance's true form, called calx. Wood and coal, which burn away to little residue, were thought to be mostly phlogiston. Iron, which corrodes but resists burning, was thought to hold very little. The theory rested on a flaw that doomed everyone who trusted it. Air played no role in it, and no quantitative experiments were run to test the idea. It simply explained why burning objects appear to grow lighter and lose something. John Mayow had already glimpsed the truth and was ignored for it. Mayow refined Boyle's proof that air is needed for combustion by showing fire requires only one part of air, which he named spiritus nitroaereus. He placed a mouse or a lit candle in a sealed container over water and watched the water rise to fill one-fourteenth of the air's volume before the subject died or the flame went out. His work failed to fit the prevailing theory and was set aside.

• 4. Three Men And A Gas

  ›

  Carl Wilhelm Scheele, a Swedish pharmacist, produced oxygen by heating mercuric oxide and various nitrates in 1771 and 1772. He could not interpret his results within phlogiston theory, so he delayed publishing until 1777, calling the gas fire air. On the 1st of August 1774, the British clergyman Joseph Priestley focused sunlight on mercuric oxide in a glass tube and liberated a gas he named dephlogisticated air. Priestley noticed candles burned brighter in it and that a mouse stayed more active and lived longer while breathing it. After inhaling it himself, he wrote that his breast felt peculiarly light and easy for some time afterward. His findings appeared in 1775 in a paper titled An Account of Further Discoveries in Air. Neither man understood he held a new element. Priestley clung to the phlogiston framework in his very naming of the gas. The French chemist Antoine Lavoisier later claimed independent discovery, though both rivals had reached out to him. Priestley visited Lavoisier in October 1774 and described the experiment in person. Scheele sent Lavoisier a letter on the 30th of September 1774 describing his own discovery, a letter Lavoisier never acknowledged. A copy turned up in Scheele's belongings after his death.

• 5. Lavoisier's Scales

  ›

  Antoine Lavoisier weighed what others only watched. Beginning in 1774, he ran the first adequate quantitative experiments on oxidation and gave the first correct account of how combustion works. Heating tin and air in a closed container, he found no overall gain in weight, but air rushed in when he opened it. The tin had gained exactly as much weight as the air that rushed back. He published this work in 1777 under the title Sur la combustion en general, proving air to be a mixture of two gases. One was vital air, essential to combustion and respiration. The other he called azote, from the Greek for lifeless, which supported neither. Azote later became nitrogen in English while keeping its older name in French. Lavoisier renamed vital air oxygene in 1777, from the Greek roots oxys, meaning sharp or acid, and -genes, meaning producer. He believed wrongly that oxygen was a component of all acids. Sir Humphry Davy showed in 1812 that hydrogen chloride is a strong acid containing no oxygen, but the name had already taken hold. English scientists resisted the foreign word even though their own Priestley first isolated the gas. A poem titled Oxygen in Erasmus Darwin's 1791 book The Botanic Garden helped the name stick.

• 6. Catching The Cold Liquid

  ›

  John Dalton assumed water's formula was simply HO, which led him to calculate oxygen's atomic mass as 8 times hydrogen's, roughly half the true value near 16. In 1805, Joseph Louis Gay-Lussac and Alexander von Humboldt showed water forms from two volumes of hydrogen and one of oxygen. By 1811, Amedeo Avogadro had reached the correct interpretation of water's composition. Turning oxygen into a liquid proved a harder race. The French brothers Quentin and Arthur Brin discovered a commercially viable reaction to make oxygen in 1879, running their process between 1886 and 1906. Raoul Pierre Pictet evaporated liquid sulfur dioxide to chill carbon dioxide, then used that to liquefy oxygen, telegraphing the French Academy of Sciences in Paris on the 22nd of December 1877. Two days later, Louis Paul Cailletet announced his own liquefying method. Each produced only a few drops. Oxygen was first liquefied in a stable state on the 29th of March 1883, by the Polish scientists Zygmunt Wroblewski and Karol Olszewski at Jagiellonian University. James Dewar produced enough liquid oxygen for study in 1891. In 1895, Carl von Linde and William Hampson independently developed the first commercially viable process. Oxyacetylene welding was first demonstrated in 1901. In 1923, Robert H. Goddard built the first liquid-fueled rocket engine, using liquid oxygen as the oxidizer, and flew a small rocket 56 meters at 97 kilometers per hour on the 16th of March 1926 in Auburn, Massachusetts.

• 7. A Molecule Of Unpaired Electrons

  ›

  Dioxygen carries two unpaired electrons, and that strange detail governs nearly everything about it. Two oxygen atoms bind in a covalent double bond with a bond length of 121 picometers and a bond energy of 498 kilojoules per mole. The molecule's ground state is a spin triplet, called triplet oxygen, because those two unpaired electrons share equal energy. Because most organic molecules have paired electron spins, triplet oxygen reacts only slowly with them, which is why the world does not burst into spontaneous flame. Liquid oxygen reveals its hidden magnetism in the laboratory. In the triplet form the molecules are paramagnetic, and a bridge of liquid oxygen can be held against its own weight between the poles of a powerful magnet. This property powers paramagnetic oxygen analyzers used in industry and medicine. Singlet oxygen, by contrast, has all its electron spins paired and reacts far more aggressively. It forms from water during photosynthesis and in the troposphere through the photolysis of ozone. Ozone itself, the allotrope known as trioxygen, absorbs strongly in the ultraviolet and shields the planet, yet near the surface it becomes a pollutant from automobile exhaust. The metastable molecule tetraoxygen, discovered in 2001, can form a rhombohedral cluster under 20 gigapascals of pressure and may one day serve as a powerful rocket oxidizer.

• 8. The Breath Of A Living World

  ›

  About 2.45 billion years ago, oxygen began building up in the atmosphere during the Great Oxygenation Event, roughly a billion years after the anaerobic organisms it would poison first appeared. Free oxygen is produced as a byproduct when light splits water during chlorophyllic photosynthesis. Some estimates credit marine photoautotrophs like algae and cyanobacteria with about 70% of the free oxygen made on Earth, with land plants making the rest. Photosynthesis and respiration run as near mirror images, one releasing oxygen and the other consuming it, so the evolution of life tracks the supply of available oxygen. Today oxygen fills 20.95% of the atmosphere by molar fraction and constitutes 49.2% of the Earth's crust by mass. The oceans are 88.8% oxygen by mass. Earth stands unusual among the planets in carrying so much atmospheric oxygen, while Mars holds only 0.1% by volume. This abundance makes oxygen the strongest biosignature in the search for life beyond Earth. It meets three criteria, since almost all of Earth's oxygen has biological origin, it persists for around a billion years, and it shows strong, distinctive absorption lines that remote telescopes can detect. Genesis spacecraft samples revealed that the Sun holds a higher proportion of oxygen-16 than the Earth, implying that an unknown process stripped oxygen-16 from the Sun's disk of material before the dust grains that became our planet ever came together.