Noble gases have full outer shells, whereas alkali metals have one unpaired electron in their outer shells.
Work Step by Step
Noble gases have shells that are full and spherically symmetric. They are therefore “satisfied” and do not tend to attract nor give up electrons easily. Group I alkali metals have a lonely s-electron in the outermost shell that spends a lot of time far away from the nucleus. The alkali metal would love to give up or share that electron to attain a closed shell configuration, so the metal is highly reactive.