Final answer:
Alkali metals have one valence electron, while alkaline earth metals have two. Alkali metals are in group 1 and alkaline earth metals are in group 2 of the periodic table, and this corresponds to their number of valence electrons. The correct option is a).
Step-by-step explanation:
The difference between alkali metals and alkaline earth metals in terms of the number of valence electrons is as follows: Alkali metals have one valence electron, while alkaline earth metals have two valence electrons.
This can be remembered by noting that alkali metals are found in group 1 of the periodic table and alkaline earth metals are in group 2.
Each alkali metal atom such as lithium and sodium has a single s electron in the outermost shell, which makes them very reactive, especially since this single valence electron can be easily lost to form a cation with a +1 charge.
In contrast, alkaline earth metals such as beryllium and magnesium have a fully completed s subshell with two electrons. When alkaline earth metals form cations, they lose these two valence electrons to assume a +2 charge, becoming isoelectronic with the nearest noble gas. Option a) is the correct one.