Question

why potasium more reactive than lithium and sodium

Answer 1

The atomic radius of potassium is greater than that of sodium. Therefore, the single valence electron that exists for all alkali metals is located farther from the nucleus for potassium than sodium. This results in less energy required to remove that valence electron from potassium than from sodium, leading to increased reactivity. Note that this trend continues as you move down Group I on the Periodic Table, meaning that Rubidium is more reactive than Potassium and Cesium is more reactive than Rubidium.