Question

why is K more reactive than Li

Answer 1

in potassium the outermost electron is better shielded from the attractive forces of the nucleus. Therefore that this outermost electron is more easily lost than it is in sodium, so potassium can be converted to ionic form more readily than sodium. Hence potassium is more reactive than sodium.

Answer 2

So potassium is more reactive than lithium because the outer electron of a potassium atom is further from its nucleus than the outer electron of a lithium atom. Francium atoms, with 7 shells, are the largest atoms in Group 1.