Why are atoms different sizes?
Atomic radius is not a single directly measurable hard boundary; several operational definitions are used, such as covalent, metallic and van der Waals radii.
The short answer
Atomic radius is not a single directly measurable hard boundary; several operational definitions are used, such as covalent, metallic and van der Waals radii.
What is really happening?
Atomic radius is not a single directly measurable hard boundary; several operational definitions are used, such as covalent, metallic and van der Waals radii.
The definition must be stated before comparing values. Periodic trends are useful, but different radius definitions should not be mixed.
The periodic table is not just a list: recurring electronic structure creates trends in atomic size, ionization energy and other properties. Trends are patterns, not magic rules, and exceptions are scientifically interesting.
A useful mental picture
Atomic size reflects a competition between nuclear attraction, electron shielding and which shells are occupied. That is why radius changes systematically—but not perfectly smoothly—across the periodic table.