Atomic Radius
A measure of the size of an atom, usually taken as half the distance between the nuclei of two bonded identical atoms.
Formula / equation
r = d(X–X) / 2
Unit
pm
In depth
Radius decreases across a period as growing effective nuclear charge pulls the same shell inwards, and increases down a group as new shells are added. Because an isolated atom has no sharp boundary, several operational definitions exist — covalent, metallic and van der Waals radii — and values must be compared like with like.
Examples in the real world
Lithium is 152 pm and fluorine only 71 pm; caesium reaches 265 pm.