Atomic radius is not precisely defined (electron cloud has no sharp boundary). Three types: covalent radius (half the internuclear distance in a homonuclear single bond), van der Waals radius (half the distance between nuclei of adjacent non-bonded atoms), metallic radius (half the internuclear distance in metallic crystal). Always: covalent < metallic < van der Waals. Across a period: radius decreases (Zeff increases, same shell, electrons pulled closer). Li (152 pm) > Be (111) > B (88) > C (77) > N (74) > O (73) > F (72). Down a group: radius increases (new shell added outweighs Zeff increase). Li (152) < Na (186) < K (227) < Rb (248) < Cs (265 pm).