Identify the element
Locate it in the periodic table
Determine its period and group
Use periodic trends to compare atomic size
Atomic size increases down a group
Atomic size decreases from left to right across a period
Compare the element with nearby elements if needed
Use atomic radius values from a reference table if available
Choose the appropriate radius type: covalent radius, metallic radius, or van der Waals radius
Use the measured or tabulated radius as the atomic size