Effective Nuclear Charge Z* = Z –  Z = nuclear charge = atomic number  = shielding = electron-electron repulsions H He Li

Slater's Rules for Effective Nuclear Charge: Calculating  1. Electrons of greater principle quantum number contribute 0. 2. If the electron being screened is an s or p electron, other electrons with the same principle quantum contribute 0.35 electrons with principle quantum number n-1 contribute 0.85 electrons with principle quantum numbers less than n-1 contribute 1.00 3. If the electron being screened is a d or an f electron, d or f electrons with the same value of n contribute 0.35, and other electrons with the same or lower value of n contribute 1.00.

Example 1: For a 2s electron in a Be atom (1s22s2, Z = 4) σ = 2(0.85) + 1(0.35) = 2.05 Zeff = 4 – 2.05 = 1.95 Example 2: For a 3p electron in a phosphorus atom (1s22s22p63s23p3, Z = 15) σ = 4(0.35) + 8(0.85) + 2(1.00) = 10.20 Zeff = 15 - 10.20 = 4.80

B C N O F

Why does Z* increase moving across a period?

Li Na K Compare with ionization energy: Li: 520 kJ/mol Na: 496 kJ/mol K: 419 kJ/mol

B C N Compare with ionization energy: B: 801 kJ/mol C: 1086 kJ/mol N: 1402 kJ/mol

Z*, Atomic Size and Pi Bonding