This is because lithium has a lower atomic number than potassium. This means that lithium has fewer electrons in its outer shell, which makes it easier for lithium to lose electrons and form positive ions. In contrast, potassium has a higher atomic number and more electrons in its outer shell, which makes it more difficult for potassium to lose electrons and form positive ions.

The reactivity of lithium and potassium can be seen in their reactions with water. Lithium reacts with water to form lithium hydroxide and hydrogen gas, while potassium reacts with water to form potassium hydroxide and hydrogen gas. However, the reaction of lithium with water is much more vigorous than the reaction of potassium with water. This is because lithium is more reactive than potassium.