Zeff = Z − σ (Slater's Rules)
The effective nuclear charge (Zeff) is the net positive charge experienced by an electron after accounting for shielding by other electrons. Slater's rules provide a method to estimate the shielding constant σ: (1) Write electron configuration in groups: (1s)(2s,2p)(3s,3p)(3d)(4s,4p)… (2) For an electron in ns/np: electrons in the same group contribute 0.35 each (except 1s: 0.30), electrons in (n−1) shell contribute 0.85, and all inner electrons contribute 1.00. (3) For nd/nf electrons: all electrons in the same group contribute 0.35, all others contribute 1.00. Zeff increases across a period (higher Z, similar shielding) explaining the trend in ionization energy, electron affinity, and atomic radius. For Na (Z=11), the 3s electron has σ ≈ 8.80, giving Zeff ≈ 2.20.