Ions in Solution

There are two important factors in deciding how ions behave in solution and how they behave in ion-exchange; the size of the ion and the charge on the ion.
For example, if an ion is highly charged, it will be strongly attracted to oppositely charged ions in the solution and polar molecules (water for example).
If the ion is small, it will be more strongly attracted to the oppositely charged ions and polar molecules, as it will be able to get in close to them.
If an ion is both highly charged and small, it can be classed as having a high charge density.
Ions with a high charge density tend to:
- Attract water molecules strongly and become very hydrated.
- Attract oppositely charged ions strongly, forming ionic lattices with large lattice energies and therefore high boiling and melting points.

Size of atoms and ions

The electrons take up the majority of the space in atoms and ions; therefore the more electrons there are in an ion or atom, the larger it will be.
For example, in Group I, the atomic radii of the atoms increases down the group as more electrons are added:
Moving across a period, the atomic radii decreases even though extra electrons are being added.
This is because the number of protons increases along the period.
As more protons are added, the nuclear pull on the electrons increases; as the electrons are being added to the same shell across each period, the atomic radii decreases as the extra charge pulls the electrons closer to the nucleus.
For example, in period three:
At potassium, the atomic radii is much larger; this is because a new electron shell is started- the fourth shell.
The fourth shell is further away from the nucleus and so potassium has a larger atomic radius.

Atoms become ions when electrons are added or taken away; this affects the size of the atomic radius.
Adding an electron to an atom (producing a negative charge) results in the ion being larger than the atom. Adding two electrons has an even greater effect.
Removing an electron results in an ion that is smaller than the atom. Removing more than one electron has an even greater effect.
The general rule is:
r²⁺ < r⁺ < r_(atom) < r^- < r^2-
This effect of charge can be illustrated in the following examples:

The above situation works for ions in the solid, liquid and gaseous phases; however, when an ionic substance is dissolved in water, the situation is different.
In water, the ions attract water molecules and become hydrated; this makes the ion bigger, as there will be a layer of water molecules around it.
The higher the charge density of the ion, the more water molecules it can attract, and therefore the bigger it becomes in water.
So, ions which are small in the absence of water become large when in the aqueous phase.
Hydration makes small, strongly attractive ions behave as if they are larger and less attracting.