Elements in the same group of the periodic table share similar physical and chemical characteristics because they have the same number of valence electrons. Valence electrons are the outermost electrons in an atom and play a crucial role in determining how an element reacts chemically.
For example, elements in Group 1 of the periodic table, such as lithium, sodium, and potassium, each have one valence electron. This commonality leads to similar reactivity patterns, such as forming similar types of ions and reacting vigorously with water. Similarly, Group 17 elements, known as halogens, each have seven valence electrons, which makes them highly reactive and leads to the formation of compounds with similar properties.
In essence, the number of valence electrons dictates how elements interact with one another, resulting in each group displaying characteristic behaviors and properties.