 Other hydrated forms, the Zundel cation , which is formed from a proton and two water molecules, and the Eigen cation , which is formed from a hydronium ion and three water molecules, are theorized to play an important role in the diffusion of protons though an aqueous solution according to the Grotthuss mechanism.The hydron itself is crucial in more general Brønsted–Lowry acid–base theory, which extends the concept of acid–base chemistry beyond aqueous solutions.Other isotopes of hydrogen are too unstable to be relevant in chemistry.