Determine the ion’s charge
Find the element’s position on the periodic table
Use the appropriate ionic radius table or chart
Compare the ion with isoelectronic species
For cations, note that radius decreases as positive charge increases
For anions, note that radius increases as negative charge increases
Compare the ion to its neutral atom
Use measured crystal structure data if available
Look up the radius in a standard reference source
Select the value that matches the ion’s coordination number and oxidation state