Why Do Elements in the Same Group Have Similar Chemical Properties?

In the periodic table, elements in the same group or family have similar chemical properties because they have the same number of valence electrons in their outer shells. When the group numbers on the periodic table are given in the older system that uses Roman numerals from IA to VIIIA, it simply is a matter of looking at the group number to know the number of an element's valence electrons.

For example, lithium is in group IA and has one electron in its highest energy level, while carbon in group IVA has four electrons. Each group has its own unique qualities. For example, group VIIIA is called the noble gas group, while VIIA is the halogen group. Similarly, group IA is the alkali metal group, and group IIA are the alkaline earth elements.