The minimum amount of energy required to remove the least strongly bonded electron from a neutral gaseous atom is known as Ionization Energy.
Ionization Energy of Alkali and Alkaline Earth Metals
Because alkali metals have low ionization energies, therefore have a great tendency to lose the single valence electron to form cations. They are so reactive that they are never found in the free state. The alkali metals show increased reactivity down the group. This is because of decrease in ionization energies down the group. This means Na is more reactive than Li, but less reactive than K. They are all so reactive that they are kept under a liquid such as kerosene oil.
Alkaline earth metals also have low ionization energies, so they also have great tendency to lose both the valence electrons to form dipositive cations. They are less reactive than alkali metals. These metals also show increased reactivity down the group. Thus, Mg is more reactive than Be but less reactive than Ca. Like alkali metals they are also kept under a liquid such as kerosene oil to prevent contact with moist air.
