About the size of the atom.
About the atomic radius,
as a starter it was mentioned (and correct me if am wrong) that:
Rcation < Ratom< Ranion (And we'll know the size of the atom based on these)
Then it was mentioned that: Radius of atom of a certain element increase if we walk straight downwards along a column in the periodic table.
And afterwards: if Z of any element is less than Z of any other one
then the radius of the first is greater than the 2nd.
How come? I really don't get it?
Any chemical, physical explanation (most importantly to the 3rd pt)