Everyone knows that acids are, well, acidic, and have a sharp taste. This is due to hydrated hydrogen ions, often written H+(aq) or H3O+(aq), though in fact a number of water molecules are actually associated with it in the hydronium or hydroxonium ion. Two representations that have been described are the Eigen structure (H9O4+; 4.1) and the Zundel structure (H5O2+; 4.2), which are present in crystals of [H9O4] [CHB11H5Br6] and [H5O2][CHB11Cl11], respectively.