In terms of electronic configuration, what the elements of a given period and groups have in common?
In general, the elements belonging to a group have the same number of valence electrons .For example, all the group 1 elements have valency one since they have only one electron in their outermost shell.
In general, the elements belonging to a period do not have same valency but their valence shell remains the same. For example, second period has 8 elements with atomic number 3 to 10 but in all of them the valence electrons are present in shell number two.
Concept Insight: For elements in a group the number of electrons present in the outermost shell is the same and therefore the elements have same valency and or elements in a period number of electrons present in the outermost shell of elements in a period increase from left to right but the shell does remains the same.