Give TWO reasons, in terms of shielding, why potassium is the

most reactive of the three elements.

Hi ... ? All three elements are in group 1 thus they have 1 valence electron which is easy to lose due to shielding. As Pottasium is larger than the rest, Pottasium's valence electron is at a greater distance from the attractive nucleus and is so removed more easily than Sodium's and Lithium's valence electrons. As it is removed more easily it requires less energy and can be said to be more reactive