Answer:
Number of valence electrons
Step-by-step explanation:
A family of elements is defined as a specific group of those elements. Groups in the periodic table are identified by columns, e. g., group 1A contains alkali metals, group 2A contains alkaline earth metals, group 7A consists of halogens, group 8A contains noble gases etc.
The group number corresponds to the valence electrons these atoms have. The number of protons for each atom is different and the level of reactivity depends on the size of the atom mainly, as well as electronegativity.