JEE Main · 2026 · 28 Jan 2026 · Shift 1 · Classification of Elements and Periodicity

Period 4 runs from potassium ($Z=19$) to krypton ($Z=36$). Across a period the atomic radius shrinks as the nuclear charge grows while electrons enter the same shell, so the first element, potassium, has the largest atomic radius. Among the elements of the period before the noble gas the radius is smallest at bromine. Sodium and chlorine lie in period 3 and rubidium in period 5, so the period-4 pair with the largest and smallest radii is potassium and bromine.