atomic radius

How big is an atom? Because an electron cloud has no hard edge, you cannot lay a ruler across an atom directly. Instead chemists measure the distance between the nuclei of two bonded atoms and split it — that gives a workable atomic radius, a stand-in for the atom's size. Atoms are tiny (radii of roughly 30 to 300 picometres, where a picometre is a trillionth of a metre), and how their size changes across the table is one of the most useful patterns in chemistry.

Two clear trends emerge. Going left to right across a period, atoms get smaller: each step adds a proton, the effective nuclear charge climbs, and the same shell of electrons is pulled in tighter. Going down a group, atoms get larger: each step adds a whole new shell further out, and although the nucleus gains protons, the added inner electrons shield them, so the outer shell sits farther away. Ionic radius follows related logic: a cation (having lost electrons) is smaller than its parent atom, sometimes dramatically, while an anion (having gained electrons) is larger because the same nuclear charge now spreads over more electrons that repel each other.

Size drives chemistry everywhere. It sets how atoms pack in crystals, which ions fit which holes in a lattice (the radius-ratio idea), how strong a bond is, and how reactive a metal is. Two honest complications worth flagging: the lanthanide contraction means the elements just after the 4f filling (like hafnium) are unexpectedly small, making second- and third-row transition metals almost the same size; and the measured radius depends on how the atom is bonded (covalent, metallic, van der Waals radii differ), so quoted numbers are context-dependent, not absolute.