The cations of the same charge increase in radius as you move down a column in the periodic table.
Explanation:
Moving down a column in the periodic table means to increase the main energy level and keeping the number of electrons in the outermost shell (the number of valence electrons).
The metals (elements in the left side of the periodic table) form positive ions, named cations, when they lose one or more valence electrons.
To depict this more clearly, consider, for example, the column 1 in the periodic table, which is the group of alkal metals: Li, Na, K, Rb, Cs, and Fr.
As you move down that column you ge the following results
Element Period Number of Main cation
(main energy level) valence electrons
Li 2 1 Li⁺
Na 3 1 Na⁺
K 4 1 K⁺
Rb 5 1 Rb⁺
Cs 6 1 Cs⁺
Fr 7 1 Fr⁺
Then, in the last column of the previous table, you see that all the cations have the same charge, because each one is formed after lossing the same number of electrons from the neutral atom (1).
Since, as you move down the column in the periodic table, the valence electrons are in higher main energy levels, which means that the size of both the neutral atom and the and the resultant cation formed after losing the valence electron are bigger than the cation of the previous level. Hence, as a general rule, the radius of the cations of the same charge increase as you move down a colum in the periodic table.
