Which of the following has a single valence electron?

Alkali metals are characterized by having a single valence electron in their outermost shell. This unique property is due to their position in Group 1 of the periodic table. The presence of only one valence electron means that alkali metals are highly reactive, as they tend to lose that electron easily in chemical reactions to achieve a stable electronic configuration, typically like that of noble gases. This is why alkali metals are known for forming strong bonds and compounds quickly. Their reactivity increases down the group, with elements like lithium, sodium, and potassium displaying significant reactivity due to this single valence electron.

In contrast, alkaline earth metals have two valence electrons, noble gases have a full set of valence electrons, and transition metals have varying numbers of valence electrons that do not follow the simple group trends associated with the alkali metals.