Step-by-step explanation:
Both sodium and potassium belong to the same group. They both have only one electon in their outermost shell. But they belong to different periods. Sodium has 3 shells while potassium has 5. As the distance between the valence electron and the nucleus is more in potassium. It experience lesser pull by the nucleus and so can give away the valence electron very easily as compared to sodium which experience a greater pull from the nucleus. Thus potassium is more reactive. Reactivity increases on moving down the group.