S orbital.
Group 1 elements have a general configuration [tex]ns^{1}[/tex], where n represents the highest occupied Principal Energy Level. For example, Lithium has the valence configuration [tex]2s^{1}[/tex] whereas Cesium has [tex]6s^{1}[/tex]. Both of them belong to Group 1 of Periodic Table.
Group 2 elements have a general configuration of [tex]ns^{2}[/tex]. For example, Magnesium has [tex]3s^{2}[/tex] as its outer shell configuration while Strontium has the same as [tex]5s^{2}[/tex].
We see that in both the cases, the outermost S orbital is being filled.
