What are #H^+# and #H_3O^+# in water in the context of acidity?
For water we could write the autoprotolysis as:
Alternatively we could write:
Both protium ion,
This acid cluster may be very short-lived, with the proton being shunted to another cluster of water molecules (if you have ever played rugby and I assume you have, think of a rolling maul).
This acidium cation is in fact a reactive species; with an electron source (a metal) it can be reduced to form dihydrogen gas: