Atoms do not have sharp edges — their electron clouds fade gradually into space, making "size" a surprisingly tricky concept. In practice, chemists define atomic radius as half the distance between the nuclei of two identical atoms bonded together (covalent radius) or, in metals, half the distance between adjacent atoms in a crystal (metallic radius). These values are measured in picometers (pm), where 1 pm = 10⁻¹² meters. The smallest atom is helium, with a van der Waals radius of about 31 pm, while the largest is cesium at about 265 pm — roughly eight times bigger. To put this in perspective, if a cesium atom were the size of a basketball, a helium atom would be the size of a tennis ball.
Atomic radius follows two clear periodic trends. Moving across a period from left to right, radius decreases. This happens because each successive element adds a proton to the nucleus and an electron to the same shell — the increased nuclear charge pulls all electrons closer, shrinking the atom. Sodium (186 pm) is much larger than chlorine (99 pm), despite both being in Period 3. Moving down a group, radius increases because each new period adds another electron shell, pushing the outer electrons farther from the nucleus. Lithium (152 pm) is smaller than sodium (186 pm), which is smaller than potassium (227 pm), which is smaller than rubidium (248 pm), and so on.
Atomic radius has real-world consequences. Smaller atoms tend to have higher ionization energies and higher electronegativities — they grip their electrons tighter. The size difference between atoms determines bond lengths and bond strengths in molecules. In materials science, atomic radius mismatches explain why some metals form alloys easily while others don't — for solid solution alloys, the Hume-Rothery rules state that atomic radii should differ by no more than 15%. Even biological processes are affected: potassium ions pass through cell membrane channels while the slightly smaller sodium ions are blocked, because the channels are precisely tuned to potassium's ionic radius.