Explain the existence of hydronium ion in aqueous solution.

(N/A) hydrogen ion $(H^{+})$ by itself is a bare proton with a very small size (radius $\approx 10^{-15} \ m$) and an intense electric field.
Therefore,the proton binds itself to a water molecule at one of the two available lone pairs on the oxygen atom via a coordinate covalent bond,forming the hydronium ion $(H_{3}O^{+})$.
This hydronium ion $(H_{3}O^{+})$ species has a trigonal pyramidal geometry. The existence of $H_{3}O^{+}$ has been detected in many compounds,such as $H_{3}O^{+}Cl^{-}$ in the solid state.
Furthermore,the hydronium ion can be further hydrated to form various ionic species such as $H_{5}O_{2}^{+}$,$H_{7}O_{3}^{+}$,$H_{9}O_{4}^{+}$,etc.
Thus,a proton does not exist independently in an aqueous solution; instead,it exists as a hydronium ion or oxonium ion.