Elements in the same family or groups have the same number of covalent electrons on their outer shells according to the shell model. So, elements who are of the same family behavior similarly chemically. For example, Ca (calcium) and Mg (magnesium) are in the same group (group 2). Since they are in group 2 they have 2 covalent electrons (electrons on their outer shell). Since they are metals they tend to lose their two electrons to anions and become cations, which most metals are. Therefore, they behavior similarly and have similar ionic behavior.
