What do elements within a group have a similar number of?
Chemical elements that are in the same group within the periodic table have the same numbers of electrons in the outermost shell.
This gives them similar reactivity.
Elements within a group have similar chemical properties because the chemical properties of element are determined by the ease which they donate or accept electrons. Elements within a group have sim,ilar chemical properties have the same number of valence electrons and they have the same number of electrons in their outermost shell.