A hydronium ion is formed when a positively charged hydrogen ion bonds with a molecule of water through a coordinate covalent bond. In this coordinate covalent bond, the oxygen from the water molecule uses both of its electrons for the bond. The hydrogen ion does not have any electrons to contribute.
Since the hydronium ion is lacking an electron, it keeps the positive charge of the hydrogen ion. It forms a molecule that is in the shape of a trigonal pyramid. The presence of a hydronium ion is one of the identifying characteristics of an acid. Any acid that is dissolved in water produces hydronium ions.