What distinguishes an alkali metal from an alkaline earth metal?

Alkali metals and alkaline earth metals have distinct characteristics primarily due to their position in the periodic table and their reactivity. Alkali metals, which are located in group 1, have a single electron in their outermost shell, making them highly reactive. This high reactivity is attributed to their tendency to lose that one electron easily, forming positive ions. In contrast, alkaline earth metals, found in group 2, have two electrons in their outer shell. While these metals are reactive as well, they do not react as vigorously as alkali metals because they require more energy to lose two electrons compared to the one electron lost by alkali metals.

The correct answer highlights this key difference: alkali metals are known for their high reactivity, while alkaline earth metals are generally less reactive, thus illustrating the contrasting chemical behavior of these two groups of elements.