# What is the polyatomic ion H3O+ called?

The hydronium ion is our conception of the acid principle of water. Another conception is ${H}^{+}$ (and please remind yourself what a conception is!). So what is the acidic species in water? It is a cluster of water molecules, 3, or 4, or 5, with an extra proton, ${H}^{+}$, associated with the water cluster to give (say) ${H}_{7} {O}_{3}^{+}$, or ${H}_{9} {O}_{4}^{+}$. In pure water, what is the counterion? And of course this cluster rapidly exchanges the ${H}^{+}$ with other clusters. It is simpler to treat this cluster, this species, whatever is it, as ${H}^{+}$ or ${H}_{3} {O}^{+}$, protium or hydronium ions.