Visualise how atomic radius changes across periods and down groups, and understand the underlying reasons.
Atomic Radius Trend
Atomic Radius (pm)
Element (increasing atomic number →)
Across a period: Atomic radius decreases because the nuclear charge increases, pulling electrons closer to the nucleus. The extra electrons go into the same shell, so shielding is minimal.
Down a group: Atomic radius increases because each successive element adds a new electron shell. The outer electrons are further from the nucleus despite the increased nuclear charge.