Charge density, the ratio of the charge to the size of the ion, is highest for small highly charged ions, namely those formed by elements at the top of groups and further to the right in the periodic table.

For Group 1 and 2, Be²⁺ has the highest charge density and Cs⁺ the lowest. 
Ions of high charge density commonly exhibit the properties below:

• covalent bonding in their compounds as the cation distorts the anion electron cloud
  BeCl₂ doesn't conduct electricity when molten.
• hydrated ion acidic as stronger M-O attraction weakens O-H
  BeCl₂ gives an acidic aqueous solution due to Be(OH₂)₄²⁺
  
• oxide (hydroxide) has both acidic and basic properties (amphoteric)
  These compounds (BeO) contain an acidic cation and a basic anion. 
  
• lesser thermal stability of compounds having polyatomic anions. 
  Lattices having a cation and anion of similar size are more stable. Decomposition to a smaller anion is more favorable for small cations.