Effective nuclear charge (Z_eff)
Electrons in multi-electron atoms don't feel the full nuclear charge because inner electrons shield outer electrons from the nucleus. The effective nuclear charge is the net positive charge experienced by an electron.
Z_eff = Z − σ
Where Z is the atomic number and σ (sigma) is the shielding constant calculated using Slater's rules.
Slater's rules (summary)
- Electrons in the same group contribute 0.35 each (0.30 for 1s).
- Electrons in the (n-1) shell contribute 0.85 each for s/p electrons.
- Electrons in (n-2) or lower contribute 1.00 each.