The elements of a group in the periodic table-—

In the modern periodic table, elements are arranged in vertical columns known as groups. Elements within the same group possess the same number of valence electrons in their outermost shell. Since chemical reactivity and bonding patterns are primarily determined by these valence electrons, elements in a group exhibit similar chemical properties. For instance, noble gases have completely filled valence shells and show little chemical activity, while alkali metals like sodium have one electron in their outermost shell, influencing their high reactivity. In contrast, atomic numbers increase sequentially across periods, not within groups. Isotopes are atoms of the same element with different mass numbers [1], and isobars are atoms of different elements with the same mass number; neither term defines the relationship between all elements within a periodic group.

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 3: Metals and Non-metals > 3.3 HOW DO METALS AND NON-METALS REA ALS REACT? > p. 46