The elements in a group of the periodic table exhibit similar chemical properties primarily because they possess the same number of outer electrons, also known as valence electrons. These outer electrons are crucial in determining how an element reacts chemically. For instance, elements in Group 1 (alkali metals) all have one outer electron, making them highly reactive and similar in their chemical behavior. Conversely, elements in Group 18 (noble gases) have a complete outer shell, which makes them largely inert. Thus, the number of outer electrons is a key factor in the similarities observed among elements in the same group.