Which group of elements is known for having one electron in their outer level?

The correct answer is Group 1. Elements in Group 1 of the periodic table are known as alkali metals, which are characterized by having one electron in their outermost energy level. This single outer electron is responsible for the reactivity of these elements, as they tend to lose that electron easily during chemical reactions. This property leads to the formation of positive ions with a +1 charge.

In contrast, Group 2 elements have two electrons in their outer level, making them less reactive than those in Group 1. Group 14 elements possess four outer electrons, which allows for different chemical bonding characteristics. Lastly, Group 18 elements, known as the noble gases, have a full outer shell of eight electrons, making them very stable and largely unreactive. Thus, Group 1 distinctly stands out for its one outer electron, explaining its high reactivity and characteristic behaviors.