This section will cover:
When a metal salt is dissolved in water, both the cation and the anion are hydrated. For transition metal ions a hexaaqua ion is formed. Water molecules act as Lewis bases (each O atom donates a pair of electrons to the metal Mn+ ion) and the metal acts as a Lewis acid, as shown below:
This arrangement of water molecules around a central metal ion is referred to as the inner hydration shell.
The hydration energy (or enthalpy) is the energy change associated with the hydration reaction (plus the hydration of the counter ion) as shown below:
Mn+(g) + H2O → [M(OH2)6]n+(aq)