Difference: Hydrogen, Hydronium, Proton

[samoyan]

I`m a little bit confused about "H+" notation. Why many books write H+ instead of hydronium H3O+? I know that H+ cannot exist by itself, it definitely bonds with another water molecule to form hydronium.

Here is an extract from Harper`s Biochemistry book:

"Protons exist in solution not only as H3O+, but also as multimers such as H5O2+. The proton is nevertheless routinely represented as H+, even though it is in fact highly hydrated."

Why they use the word proton to refer to hydronium? It`s just a positively charged ion with one proton in excess.

[victor]

I can think like this:
The pH of pure water is 7.0. It means that ions which compose water molecule (H+ and OH-) is combining altogeter forming a structure called as H2O. An addition of H+ ion to a pure water, i.e. from HCl solution, would make the concentration of H+ is excessive (depends on the volume of the water used and the concentration of acid).
This excessive H+ would bind (or get hydrated) become hydronium ions or another multimeric forms. But the point is that about the equilibrium of these ions since as far as I know that hydrated forms is easily changeable into another forms, example:
H3O+ <-> H2O + H+
so, because of these equilibrium, we can put the hydrated forms aside and focusing only to the H+ since this little thing is the one which affecting the pH change.
Oh, about the definition of hydrogen, hydronium and proton. Hydrogen is usually referred to the hydrogen atom (H) of hydrogen gas (H2), while hydronium is an ionic form of (H2O). Then, proton is defined as an ionic form of hydrogen, which is H+ or a component of atom's nucleus.

[samoyan]

Thank you very much!