Oxygen has a slightly greater density than nitrogen. Why, then, don’t these main constituents of air separate out?

GAS MOLECULES move rapidly at room temperature, with oxygen and nitrogen travelling at around 500 metres per second, so they obviously collide frequently. This allows the oxygen and nitrogen molecules to mingle and mix, rather like large numbers of people on a nightclub dance floor, in a process known as diffusion. Convection, the transfer of heat within the atmosphere, also plays an important role in this gas mixing process.

Gas mixing is a spontaneous process. This means that if you had a container with two compartments separated by a barrier, with one compartment containing pure nitrogen and the other pure oxygen, the two gases would automatically mix or diffuse as soon as the barrier was removed.

A change in the ratio of oxygen to nitrogen would be expected in a hypothetical quiescent atmosphere. However, constant mixing occurs in the real atmosphere, driven by the Earth’s rotation and by differences in density between hot air at the Earth’s surface and colder air higher up.

Up to altitudes of between 80 and 120 kilometres this mixing results in a uniform concentration of oxygen and nitrogen – which respectively make up approximately 21 per cent and 78 per cent of the atmosphere.

This region is known as the homosphere. Partial stratification of the two gases does occur above 120 kilometres, in the heterosphere, where the density of air is much lower than at the surface and the efficiency of bulk mixing processes is reduced.

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If there were no circulation in the atmosphere, the oxygen would tend to concentrate in the lower strata. This process would take millions of years once circulation ceased because molecules of oxygen (and, indeed, nitrogen) are constantly colliding with other molecules. This means it would take a long time for a particular molecule to fall from its starting point to the ground. Once it hit the ground, it would bounce and eventually rise again to a great height, only to fall again. This would be repeated frequently if no other variable, such as temperature, changed.

Although the individual molecules continue to travel up and down, each ‘species’ of oxygen and nitrogen would eventually reach an equilibrium distribution of molecules per unit volume as a function of height. This species density will decrease with height by an amount that depends on the molecular weight of the species. So, the oxygen would fall off with height slightly faster than the nitrogen. At high altitudes, the air would become richer in nitrogen, but then other gases such as water vapour, neon, methane, helium and hydrogen would dominate.

In fact, atmospheric circulation and turbulence prevents this from happening in the lower atmosphere. But in the very high atmosphere there is not much circulation and the composition does become dominated by atomic oxygen. Above 600 kilometres this is superseded by helium, and eventually by atomic hydrogen.

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. Molecular geometry

This describes the overall shape of a molecule in terms of how the atoms inside it are arranged. Examples of simple structures are linear molecules like carbon dioxide (O=C=O) and tetrahedral molecules like methane, which consists of a carbon atom with four hydrogen atoms surrounding it at the corners of a tetrahedron.

Trigonal-bipyramidal molecules are shaped like two pyramids back to back, while octahedral molecules have a shape like an eight-sided solid. Octahedral molecules include the compound sulphur hexafluoride (SF6).

‘Isomers’ are compounds that have the same chemical formula but different molecular structures. For instance, the sugar fructose is an isomer of glucose – they have the same formula C6H12O6, but their atoms are arranged in different ways. Sometimes, two isomers are mirror images of each other, in which case the molecule is said to be ‘chiral’ and the two mirror-image forms are called enantiomers. Chiral molecules include most amino acids (which are the building blocks of proteins).