Use the periodic table trend: atomic radius generally increases down a group and decreases across a period from left to right
Identify the element’s position in the periodic table
For a neutral atom, use the covalent radius, metallic radius, or van der Waals radius depending on the context
For a simple estimate, compare the element with nearby elements whose atomic radii are known
If using measured data, take half the distance between the nuclei of two identical bonded atoms
For ionic species, use ionic radius instead of atomic radius
For hydrogen-like atoms, use the Bohr model formula: radius = n²a₀ / Z
For multi-electron atoms, use effective nuclear charge trends to estimate relative size
Check a reliable chemistry data table for the specific atomic radius value if exact data is needed