Which metals are more reactive: the alkali or the alkaline earth metals? Explain your answer by using concepts of atomic radius and ionization energy.

Alkali (or Group I) metals are more reactive than alkaline earth (or Group 2) metals. The reason is very simple, alkali metals have only 1 valence electron as compared to the 2 valence electrons in case of alkaline earth metals. It is relatively easier to lose or remove 1 electron as compared to 2 electrons. This makes alkali metals more reactive than alkaline earth metals. 

In terms of ionization energy, alkali metals have lower ionization energy than alkaline earth metals. Ionization energy is the amount of energy required to remove the valence or most loosely bound electron. Because of lower ionization energy, alkali metals find it easier to lose an electron. In terms of atomic radius, alkali metals have a higher atomic radius than alkaline earth metals. However, alkali metals have to lose only 1 electrons, while alkaline earth metals will find it relatively harder to lose the second electron.

Hence alkali metals are more reactive than alkaline earth metals.

Hope this helps.