When a bond gets too extreme

Following this principle, chemists have also found it convenient to sub-divide chemical bonds into types such as covalent bonds (in which the bonding electrons are localised near the atoms of the bond and the electronic charge is divided approximately equally between them), ionic bonds (in which the charge is also localised, but resides largely on one, more electronegative atom) and metallic bonds (in which the bonding electrons all enter delocalised conduction bands that permeate the material). Regardless of their mathematical inexactitude, such concepts generally work well under the conditions at which everyday chemistry is observed and conducted. They explain why ionic compounds usually dissolve in polar solvents like water to produce conductive liquids, whereas giant covalent structures are less soluble. They explain the ductility and electrical conductivity of metals.

But models that are ‘useful’ under one set of conditions are not necessarily so under others, and chemical bonding can look very different at unfamiliar temperatures and, especially, pressures.