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, as they are involved in bonding with other atoms. For example, elements in Group 1 (the alkali metals) all have one outer electron, which makes them highly reactive and gives them similar properties. Conversely, elements in Group 18 (the noble gases) have a complete outer shell of electrons, making them largely inert. Thus, the number of outer electrons is a key factor in the similarities observed among elements in the same group.