Which group of elements is known for having one valence electron and high reactivity?

The group of elements known for having one valence electron and high reactivity is the alkali metals. These elements are located in Group 1 of the periodic table, and their characteristic of having a single electron in their outermost shell makes them highly reactive, particularly with nonmetals and water. When they lose that one valence electron, they achieve a stable electron configuration, which drives their tendency to react.

In contrast, alkaline-earth metals, found in Group 2, have two valence electrons and, while they are reactive, they are not as reactive as alkali metals. Halogens, located in Group 17, possess seven valence electrons and are highly reactive as well, but they react differently, typically gaining an electron rather than losing one. Noble gases, situated in Group 18, are known for their stability and lack of reactivity due to having a full outer shell, making them the least reactive of all element groups. Thus, the alkali metals’ unique electron configuration and resulting reactivity clearly identifies them as the group mentioned in the question.