Use the periodic table trend: atomic radius generally increases down a group and decreases across a period from left to right
Measure the distance between the nuclei of two identical bonded atoms and divide by two
For nonbonded atoms, use X-ray crystallography or electron diffraction data to estimate atomic size
Choose the appropriate radius type: covalent radius, metallic radius, or van der Waals radius
For covalent radius, use half the bond length in a homonuclear molecule
For metallic radius, use half the distance between adjacent metal atom nuclei in a crystal lattice
For van der Waals radius, use half the distance between two nonbonded atoms in closest contact
Compare measured atomic size with known tabulated values for the element
Account for ionic charge if determining ionic radius instead of atomic radius
Use spectroscopic or computational methods when experimental measurements are unavailable