Elements in the same group of the periodic table have similar physical and chemical properties because they have the same number of electrons in their outermost shell, known as valence electrons. These valence electrons largely determine how elements react chemically and influence their physical properties. Since elements in the same group share this common valence electron configuration, they exhibit similar behavior in reactions and comparable physical traits. For example, all elements in Group 1 have one valence electron, making them highly reactive and prone to similar chemical reactions, such as reacting vigorously with water. Similarly, Group 17 elements have seven valence electrons and tend to gain one electron to achieve a stable configuration, reacting in a characteristic way like forming salts. Noble gases in Group 18 have full outer shells, making them stable and unreactive. In summary, the key reason elements in the same group have similar properties is their identical valence electron count, leading to similar chemical reactivity and physical characteristics.