Main group elements that are located within the same group (also known as a family, which is a column on the Periodic Table) contain the same number of valence electrons. The valence electrons of an atom are those that are involved in chemical bonds. Thus, the valence electrons have to do with the reactivity of an element. Therefore, elements found in the same group (family) within the periodic table have similar reactivities. Examples of the reactivities of elements within select families in the periodic table are described below.
IA: The alkali metals contain one valence electron, which they tend to lose when creating a chemical bond.
VIIA: The halogens contain seven valence electrons. They tend to gain the one electron that is lost by an alkali metal.
VIIIA: These elements are known as the noble gases. They contain eight valence electrons. Atoms that contain eight valence electrons are considered to be stable. Therefore, the noble gases tend to have little to no reactivity.