hydronium

The case of hydronium ions (H₃O⁺) is an example of this. In the gase phase, hydronium is a weak acid. The data in Table 3.10 indicates that bare hydronium ions have a higher electron affinity than most metal surfaces, so a bare hydronium ion should be neutralized when it absorbs on a bare metal surface. However, a hydrated hydronium ion is a stronger acid.