The hydrogen ion (proton)

The ionization energy of hydrogen (defined for reaction 1.1) is 1312 kJ mol^‑1, a value that is high enough to preclude the existence of H⁺‏ ions under ordinary conditions.

          (1.1)

However, as we discussed in Chapter 6, the hydrated proton or oxonium ion, [H₃O]⁺‏, is an important species in aqueous solution; Δ_hydH^o)H‏;g( = ^_1091 kJ mol^_1 (see Section 6.9). The [H₃O]⁺‏ ion (1.1) is a well-defined species which has been crystallographically characterized in various salts. The ions [H₅O₂]‏ (Figure 1.1) and ½H₉O₄_‏ have also been isolated in crystalline acid hydrates. The [H₅O₂]‏ and [H₉O₄]‏ ions are members of the general family of hydrated protons [H(H₂O)n]‏ (n =1 to ≈ 20) and we return to these ions when we discuss hydrogen bonding in Section 9.6.

(1.1)

When crystals of a compound are grown from a solvent, they may contain solvent of crystallization; if the solvent is water, the compound is a hydrate. The formula of the solvated compound shows the molar ratio in which the solvent of crystallization is present, e.g. CuSO₄.5H₂O, copper(II) sulfate pentahydrate or copper(II) sulfate–water (1/5).

Fig. 1.1 The structure of [H₅O₂]⁺ determined by neutron diffraction in the compound [V(H₂O) ₆][H₅O₂][CF₃SO₃]₄. [F.A. Cotton et al. (1984) J. Am. Chem. Soc., vol. 106, p. 5319.]