The elements within a group or family in the periodic table have similar chemical properties and the same valence shell electronic configuration because they have the same number of valence electrons.
The periodic table is organized based on the electronic structure of atoms, specifically the arrangement of electrons in their energy levels or shells. Each row in the periodic table represents a period, and each column represents a group or family. Elements in the same group have the same number of valence electrons, which are the electrons in the outermost shell of an atom.
The valence shell electronic configuration determines the chemical behavior of an element, including its ability to form bonds and participate in chemical reactions. Elements with the same number of valence electrons have similar outer shell configurations, resulting in comparable chemical properties. These similarities arise because elements with the same number of valence electrons have similar tendencies to gain, lose, or share electrons in order to achieve a stable electron configuration, such as the noble gas configuration.
For example, all the elements in Group 1 (the alkali metals) have one valence electron in their outermost shell. This gives them similar reactivity, as they readily lose that one electron to form a +1 ion. Similarly, elements in Group 17 (the halogens) have seven valence electrons and tend to gain one electron to achieve a stable configuration with eight valence electrons, forming a -1 ion.
In summary, elements within a group of the periodic table have the same valence shell electronic configuration because they have the same number of valence electrons, which leads to similar chemical properties and reactivity.