Calcium is less reactive than Potassium due to its higher ionization energy.
Work Step by Step
Potassium has a lower effective nuclear charge than Calcium, which results in it having a lower ionization energy. Additionally, Potassium, unlike Calcium, can have the stable, full valence shell if it loses one electron. Furthermore, Potassium is far more reactive than Calcium because Potassium can gain a full valence shell through losing an electron, making it extremely volatile.