How is H3O+ ion formed? Formation of Hydronium ion When water (H2O) combines with H+ ion ,results in the formation of hydronium ion through coordinate covalent bond. In H2O ,Oxygen is a central atom having 6 electrons in its valence shell , 2 electrons are shared with 2 H atoms and 2 lone pair(unshared) electronsFormation of Hydronium ion (H3O+) – Coordinate Covalent Bond Example – Digital Kemistry