Answer :
Answer: Potassium is the element which is less reactive than rubidium and more reactive than Calcium.
Explanation: Reactivity of an metal is defined as the tendency to loose electrons easily. In a periodic table, the reactivity increases down the group as size of the metal increases. The reactivity decreases across a period as the size of the metal decreases.
Rubidium lies in group 1, which has elements in the series: H, Li, Na, K, Rb, Cs and Fr.
And Calcium lies in period 4 which has elements in the series: K, Ca, (1st transition series), Ga, Ge, As, Se, Br and Kr.
So, the element to have reactivity more than rubidium and less than Calcium must be Potassium (K).
