How hydronium ion is formed?

A hydronium ion (H₃O⁺) is produced when a hydrogen ion (H⁺), or proton, from an acid combines with a water molecule (H₂O). The oxygen atom in water donates its lone pair of electrons to bond with the H⁺ ion, forming a covalent bond and generating the positively charged hydronium ion. This reaction is represented as:

 H⁺ + H₂O → H₃O⁺

How the reaction occurs step-by-step:

1. Acid ionizes- HA ⇌ H⁺ + A⁻ (More exactly in water: HA + H₂O ⇌ H₃O⁺ + A⁻)
2. Encounter/H-bonding- H⁺ (or HA⋯H₂O) is stabilized near a water molecule via hydrogen bonding.
3. Lone-pair donation
   The oxygen on H₂O donates a lone pair to the proton. This is a Lewis base → Lewis acid interaction.
4. Proton transfer- A coordinate (dative) O–H bond forms and the proton shifts to water: H⁺ + H₂O → H₃O⁺ (equivalently, HA + H₂O → H₃O⁺ + A⁻)
5. Product stabilization- The hydronium ion (H₃O⁺) forms (trigonal pyramidal). It is immediately solvated by surrounding waters via H-bonding.
6. Dynamic equilibrium/proton hopping (context)- In bulk water, the excess proton can relay through the H-bond network (Grotthuss mechanism), rapidly exchanging between H₃O⁺ and nearby H₂O.