Question

What is the `"acidium species"` in water?

Answer 1

Because `H^+` is a conception, as is `H_3O^+`. The actual species (as far as anyone knows) is a cluster of water molecules `(OH_2)_n`, with an extra `H^+`, to give `(O_nH_(2n+1))^+`.

Explanation:

Water molecules are exceptionally proton dense (and also dense in terms of molecules); the charge can pass from molecule to molecule in the cluster rather than an actual proton; in a sense this is proton tunneling. The same type of charge transfer can occur with the hydroxide ion, `OH^-`. If you have ever played rugby, consider a maul, where the forwards pass the pill from hand to hand while moving forward (and here the ball is the extra `H^+`; of course rugby is a macroscopic phenomenon).

Other ions of course cannot tunnel by charge transfer, and must physically transfer across the solution.