For main-group elements, are outer electron configurations similar or different within a group? Within a period? Explain.

Same within the groups ...

Different within the periods ...

Explanation:

The outer configuration is same with a group because of their arrangement in the periodic table. If we see group 1a of alkalies, all of them have 1 electron in their outer shell and this order goes on increasing up-to noble gases that have 8 electrons in their outer shell each.

This arrangement is called periodicity of the periodic table and elements are arranged according to the periodic law.

Along a period, these properties of outer configuration goes on increasing than within the group.