Hydronium and Hydroxide Ions

If you refer to the following Table 1 Ions in Solution (Electrolytes) you will see that pure water does conduct some electrical current, albeit much less than-even the weak electrolytes listed there.

This is because water itself is a very weak electrolyte. It ionizes to hydrogen ions and hydroxide ions to an extremely small extent:

`H_2O(l) ⇌ H^(+)(aq) + OH^(-)(aq)`      (1a)

A hydrogen ion, H⁺, is a hydrogen atom which has lost its single electron; that is, a hydrogen ion is just a proton. Because a proton is only about one ten-thousandth as big as an average atom or ion, water dipoles can approach very close to a hydrogen ion in solution. Consequently the proton can exert a very strong attractive force on a lone pair of electrons in a water molecule—strong enough to form a covalent bond:

The `H_3O^(+)` formed in this way is called a hydronium ion. All three of its `O-H` bonds are exactly the same, and the ion has a pyramidal structure as predicted by VSEPR theory. To emphasize the fact that a proton cannot exist by itself in aqueous solution, this chemical reactions is often rewritten as:

`2H_2O(l) ⇌ H_3O^(+)(aq) + OH^(-)(aq)`