Which will have a greater charge density Na+ or K+; Na+ or Al3+. Charge density formula (charge/radius)?

Answer 1

Na+ has a greater charge density than K+. Both ions have the same charge, but Na+ is a smaller ion than K+.

Al3+ has the greater charge density because of it's much higher charge. (For that matter, it's also a much smaller ion than Na+.)

As far as a formula goes, the charge density would be the charge divided by the "volume" of the ion. For practical purposes, it works to think of it as charge divided by radius as you suggest. Increasing the charge or decreasing the size of the ion increases the charge density.

Answer 2

K+
Bc the dinsty is greater