Question

explain why potassium is more reactive than lithium and chlorine less reactive than fluorine .

Answer 1

Potassium and Lithium are metals and they have the tendency to loose electrons.Lithium is smaller in size than potassium . so valence electrons are more tightly held in Li than K. Due to larger size of K the valence electrons are loosely held and easy to remove the electron from K.Hence K is more reactive.
Fluorine and Chlorine are non-metals.They have the tendency to gain electron. Due to smaller size of fluorine it can hold the incoming electron more tightly than Cl hence more reactive than Cl.