Atomic radius: definition in chemistry
Atomic radius: A measure of the size of an atom, usually taken as half the distance between the nuclei of two identical atoms bonded together.
Atoms have no sharp outer edge, so their size is worked out from the spacing of nuclei in molecules or solids, typically in picometers (1 pm = 10⁻¹² m). Across a period, atomic radius decreases because each added proton pulls the electrons of the same shell closer. Down a group it increases, because each new period adds an electron shell farther from the nucleus.
Examples
- Carbon — about 77 pm, half the C–C bond length in diamond
- Chlorine — about 99 pm, half the 199 pm Cl–Cl bond
- Sodium atoms are larger than chlorine atoms, though both are in period 3
- Cesium — about 265 pm in the metal, one of the largest atoms
On the periodic table
Atomic radius increases down each group and decreases from left to right across each period, so the smallest atoms are at the top right and the largest at the bottom left.