Define Hydronium and Hydroxyl ions.

Hydrogen ions, H⁺ also known as proton has very small size and greater electric field. Hence, it readily combines with water molecule to form Hydronium ion i.e. H₃O⁺.

For example: when a hydrogen chloride HCl dissolves in water, it gives a hydrogen ion to a neighboring water molecule as shown below:
HCl (g) + H₂O (l) ————-> H₃O⁺ + Cl^– (aq)

In aqueous solution, the Hydronium ion is itself strongly hydrated in solution to give species like H₅O⁺₂, H₇O⁺₃ and H₉O⁺₄.
Similarly, the hydroxyl ion is hydrated to give ionic like H₃O^–₂, H₅O^–₂, H₅O^–₂.