Solvation is the attraction and association of molecules of a solvent with molecules or ions of a solute. As ions dissolve in a polar solvent they spread out and become surrounded by solvent molecules. The smaller the ion, the more solvent molecules are able to surround it and the more it becomes hydrated. Similarly, the higher the charge on the ion the more solvent molecules it attracts. Solvent molecules have the effect of pulling apart the electron clouds that surround an ion's nucleus, and the higher the charge and the smaller the ion (these factors combined are known as charge density) then the larger an ion becomes when dissolved.
For solvation to occur, energy is required to release individual ions from the ionic lattices they are present in. This is necessary to break the attractions the ions have with each other and is equal to the solid's lattice enthalpy (the energy released at the formation of the lattice as the ions bonded with each other.) The energy for this comes from the energy released when ions of the lattice associate with molecules of the solvent. Energy released in this form is called the enthalpy of solvation (or with water as the solvent, the enthalpy of hydration.) Again, the amount of energy released depends on the degree of attraction and the number of solvent molecules the ion associates with, and thus increases with charge density.
Enthalpy of solvation can help explain why solvation occurs with some ionic lattices but not with others. The difference in energy between that which is necessary to release an ion from its lattice and the energy given off when it combines with a solvent molecule is called the enthalpy change of solution. A negative value for the enthalpy change of solution corresponds to an ion that is likely to dissolve, whereas a high positive value means that solvation will not occur. It is possible that an ion will dissolve even if it has a positive enthalpy value. The extra energy required comes from the increase in entropy that results when the ion dissolves. The introduction of entropy makes it harder to determine by calculation alone whether a substance will dissolve or not.