How do we make liquid nitrogen

Nitrogen condenses to form a liquid at — 196°C (77 K), which is so much lower than the temperature of 373.15 K at which water condenses that we suspect a different physicochemical process is in evidence. Below —196 °C, molecules of nitrogen interact, causing condensation. That there is any interaction at all should surprise us, because the dipoles above were a feature of heteronuclear bonds, but the di-nitrogen molecule (N=N) is homonuclear, meaning both atoms are the same. 

We need to invoke a new type of interaction. The triple bond between the two nitrogen atoms in the di-nitrogen molecule incorporates a huge amount of electron density. These electrons are never still, but move continually so, at any instant in time, one end of a molecule might be slightly more negative than the other. 

The electron density changes continually, so induced dipoles never last more than about 10-11 s. Nevertheless, they last sufficiently long for an interaction to form with the induced dipole of another nitrogen molecule nearby. We call this new interaction the London dispersion force after Fritz London, who first postulated their existence in 1930. 

A homonuclear molecule comprises atoms from only one element homo is Greek for same. Most molecules are heteronuclear and comprise atoms from several elements het-ero is Greek for other or different.  

Dipoles are usually a feature of heteronuclear bonds, although a fuller treatment needs to consider the electronic environment of atoms and groups beyond the bond of interest. 

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