Please explain what is meant by “charge density” in the cont…

Please explain what is meant by “charge density” in the context of atomic/ionic characteristics and how “ionic potential” and “electronegativity” are synonymous or different from this concept.  Please use concept of ionic potential to explain why some elements form exchangeable ions while some participate in hydrolysis reactions, as well as why some hydrated cations are more readily exchanged from sorption sites than others.