Determine the atom’s size definition you need: covalent radius, metallic radius, van der Waals radius, or atomic radius from a model
Use experimental data from X-ray diffraction, electron diffraction, or spectroscopy when available
Measure the distance between nuclei of two identical bonded atoms
Divide that internuclear distance by 2 to get the atomic radius
For nonbonded atoms, use the van der Waals distance and divide by 2
For ions, use ionic radius tables instead of neutral-atom radius values
For hydrogen-like atoms, use the Bohr model formula
Use periodic table trends to estimate radius if exact data is unavailable
Compare values from reference tables for the element and bonding type
Convert units consistently, usually from picometers or nanometers to the desired unit