The element that is more reactive than lithium and magnesium but less reactive than potassium is sodium (Na).

Here's why:

* Reactivity in Alkali Metals: Alkali metals (Group 1) become more reactive as you move down the periodic table. This is because the outermost electron is further away from the nucleus and more easily lost, leading to a greater tendency to form cations.

* Order of Reactivity: The order of reactivity for the alkali metals is:

* Lithium (Li) < Sodium (Na) < Potassium (K)

Therefore, sodium sits in the middle, being more reactive than lithium and magnesium but less reactive than potassium.