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