Why is ionization energy for calcium greater than that for potassium?

1 answer

The two electrons of Ca are in the same orbital as the one electron from K so the distance is the same from the outside shell to the nucleus.
On Ca there are 18 electrons between the two outside electrons and the inner 20+ nuclear charge so the outside electrons have a net of 2+ charge attracting them (20+ nuclear - 18- shieding electrons between = 2+ net charge.
For K there are the same number of shielding electrons (18) so the net attraction for the one outside electron of K is +19 - 18- = 1+ net charge. So the attraction between the outside electrons is higher for Ca which makes the ionization energy higher for Ca.