Use the formula: (Z_{text{eff}} = Z – S)
Identify (Z), the atomic number
Determine (S), the shielding constant
Estimate (S) using Slater’s rules
For same-shell electrons, use the appropriate shielding value from Slater’s rules
For inner-shell electrons, add their shielding contributions
Subtract total shielding from atomic number
For a quick estimate, use (Z_{text{eff}}) as the net positive charge felt by the electron
For valence electrons, calculate shielding from core electrons first
Compare (Z_{text{eff}}) across elements by noting that it generally increases from left to right across a period
Compare (Z_{text{eff}}) within a group by noting that it changes less dramatically down a group