ABSTRACT
The most striking change in dimension would come if the hydrogen atom were to lose its electron to become the positive ion, H+. This is the bare proton with a radius of about 10~3pm and is a hundred thousand times smaller than any other ion (compare Li+, radius 60pm). The charge density on the proton is thus enormously higher than on any other chemical species and it would have a powerful polarizing effect on any other molecule in its neighbourhood. As a result, the free proton has no independent existence in any chemical environment and always occurs in association. Thus, if an acid dissociates in water, H3O+
and not (except as a shorthand) H+ is formed, and this 'hydrogen ion' or 'hydroxonium' ion is then further solvated just as any other cation. A similar situation holds for any other protonic solvent, as discussed in Chapter 6 (although it must be noted that in the Brônsted definition of an acid it is the proton which is transferred). In water, the proton is in a rapidly changing environment and interacting with varying numbers of water molecules. The type of species present in solution is illustrated by the various hydrated protons which have been isolated and characterized. Examples are known where the proton interacts with 2, 3, 4 or even more H20 molecules, including HgO^, shown in Fig. 9.1, which was extracted from an aqueous acid solution into an immiscible organic base. A simpler species, where the proton is linked to two water molecules, was isolated in a solid complex, see Fig. 9.2.
