Element GroupsElements in a group share a common number of valence electrons. For example, all of the elements in the alkaline earth group have a valence of 2. Elements belonging to a group typically share several common properties.
Element PeriodsElements in a period share a highest unexcited electron energy level. There are more elements in some periods than others because the number of elements is determined by the number of electrons allowed in each energy sublevel.
Learn MoreGroups and Periods
Overview of the Element Groups
Periodic Properties of the Elements