WAEC - Chemistry (2001 - No. 1)
Elements which belong to the same group in the Periodic Table are characterized by
difference of +1 in the oxidation numbers of successive members
presence of the same number of outermost electrons in the respective atoms
difference of 14 atomic mass units between successive members
presence of the same number of electron shells in the respective atoms
Erläuterung
Elements in the same group have the same number of valence electron in their outermost shell while elements in the same period have the same number of shells.