Why do elements in the same group have similar physical and chemical properties?

Elements in the same group (or column) of the periodic table have similar physical and chemical properties because they have the same number of valence electrons. Valence electrons are the outermost electrons in an atom, and they are primarily responsible for the chemical behavior of an element.

Since elements in the same group have the same number of valence electrons, they tend to exhibit similar patterns of reactivity and bonding. For example, elements in Group 1 (such as lithium, sodium, and potassium) all have one valence electron, which makes them highly reactive and likely to form ions with a +1 charge. Similarly, elements in Group 17 (such as fluorine, chlorine, and bromine) all have seven valence electrons and tend to form ions with a -1 charge or share electrons to complete their valence shells.

Additionally, elements in the same group often have similar atomic sizes and ionization energies due to the same number of electron shells and similar effective nuclear charges. This similarity in atomic structure further contributes to their similar chemical behavior.