Oxygène

Because O₂ molecule, although a powerful oxidant, has a high kinetic barrier: it needs a spark, an enzyme or high temperature to start reacting. At 20°C and without a catalyst, neither paper nor our skin burns in air. Cells use enzymes that release their energy step by step in respiration; a flame releases it all at once. It is a sleeping oxidant that only wakes with sufficient activation.

No: breathing pure O₂ at normal pressure irritates lungs in hours and at high pressure causes convulsions (Paul Bert effect known in diving). Air with its 21% is an evolutionary balance; more oxygen generates free radicals that damage tissues. In medicine it is dosed with precision and in technical diving partial pressures are limited. More is not better.

N₂ and O₂ molecules in air scatter sunlight, and blue, with short wavelength, scatters much more than red (Rayleigh scattering). That is why the daytime sky is blue and the sunset turns red, when light passes through more air thickness. Without an atmosphere there is no colour: the sky seen from the Moon is black even in full daylight.

Because rusting means donating electrons to oxygen, and each metal has a different 'generosity': iron, magnesium or sodium donate them easily and corrode; gold and platinum retain them and remain intact for millennia. The reactivity series orders metals by this tendency. Aluminium is a curious exception: it oxidises instantly, but its transparent oxide seals and protects it.

The planetary poisoning that created our air: about 2.4 billion years ago, photosynthetic cyanobacteria filled the atmosphere with oxygen, toxic to almost all anaerobic life of that time. It caused a mass extinction, oxidised the iron of the oceans (current mines) and opened the era of aerobic respiration, 18 times more efficient. We owe the air to a microbial catastrophe.

They are the same chemical family with different numbers of oxygen atoms: O₂ that we breathe (double bond, stable molecule), O₃ or ozone (three atoms, aggressive oxidant that forms the stratospheric shield) and H₂O₂ or hydrogen peroxide (with a weak O–O bond that releases active oxygen when it breaks). From vital air to pharmacy disinfectant only the molecular architecture of oxygen changes.

No on a human scale: the atmospheric reserve is equivalent to thousands of years of respiration and photosynthesis, and burning all fossil fuels would consume less than 2%. The real danger is not the global amount but the local: eutrophication that leaves seas without oxygen, unventilated interiors or loss of forests. Taking care of oxygen is taking care of its cycles, not counting molecules.