H3O+ is a chemical formula that represents a positively charged hydronium ion. It is formed when a water molecule (H2O) gains a hydrogen ion (H+) through the process of protonation. In an aqueous solution, water molecules can undergo autoionization, where a small fraction of them naturally dissociate into hydronium ions and hydroxide ions (OH-). The hydronium ion, H3O+, is essentially a water molecule with an extra proton attached to it.