Question

why are the alkaline earth metals less reactive than the alkali metals?

Explanation:

Alkaline earth metals have smaller atomic radii and higher ionization energies compared to alkali metals. Alkali metals have one valence electron, while alkaline earth metals have two. Removing one electron (as in alkali metals) is easier than removing two (as in alkaline earth metals). Also, the effective nuclear charge is higher for alkaline earth metals (due to more protons in the nucleus for the same period), which holds the electrons more tightly.

Answer:

Alkaline earth metals are less reactive than alkali metals because they have higher ionization energies (harder to lose electrons as they have two valence electrons to lose compared to one for alkali metals) and smaller atomic radii (due to higher effective nuclear charge), making it more difficult for them to give up electrons in chemical reactions.