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Very often a hydronium ion H₃O⁺ attaches itself to another water molecule, creating H₅O₂⁺.

This is called a 'Zundel cation', named after Georg Zundel, a German expert on hydrogen bonds.

In this picture, the H⁺ in the middle looks more tightly connected to the water at right than the water at left. But in fact it should be completely symmetrical. At least, that’s the theory of how a Zundel cation works!

However, the Zundel cation is not the end of the story. When you've got an extra proton around, it can really attract a *lot* of nearby water molecules. So read on....

(3/n)