Explain that elements in the same group have the same number of outer-shell electrons.

Elements in the same group of the periodic table share the same number of outer-shell electrons, which significantly influences their chemical properties. For example, Group 1 elements, known as alkali metals, each have one electron in their outer shell, making them highly reactive and similar in behavior. Conversely, Group 7 elements, the halogens, possess seven outer-shell electrons, leading to their tendency to gain an electron during reactions. This similarity in outer-shell electron configuration is crucial for predicting how elements will interact chemically.